diff --git a/day_5/example.txt b/day_5/example.txt
new file mode 100644
index 0000000..d4d4441
--- /dev/null
+++ b/day_5/example.txt
@@ -0,0 +1,28 @@
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\ No newline at end of file
diff --git a/day_5/input.txt b/day_5/input.txt
new file mode 100644
index 0000000..73049e7
--- /dev/null
+++ b/day_5/input.txt
@@ -0,0 +1,1378 @@
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+61,37,96,69,33,28,87,26,82,19,79,35,77,67,54,39,62,97,23,24,99,12,21
+26,82,79,35,12,47,84
+55,93,86,59,84
+93,28,52,43,86,78,61,57,13,68,75,18,15,64,53
+74,55,47,49,59,13,64,68,57,86,18,94,52
+59,21,86,99,18,57,49,74,12,66,84,31,24,68,46
+82,28,13,78,52,14,33,86,46
+68,57,86,94,27,37,96,87,35
+57,46,84,94,75,15,47,64,12,53,59
+66,33,74,35,88,69,12,19,79,28,87,54,99
+77,82,35,99,24,31,62,93,21,39,67,79,54,19,66,23,97
+12,67,23,39,47,62,66,99,88,21,24,74,64,59,53
+39,19,62,87,77,35,14,26,28,96,99,23,61,24,66,27,97,67,69,33,54,79,82
+13,64,46,43,15,18,94,27,14,61,37,96,69,33,28,87,82
+19,62,47,49,84
+47,23,53,46,31,93,54,64,67
+18,93,21,57,15,84,68,64,86,46,88,13,24,12,53
+33,87,82,79,67,39,24,66,47
+94,75,82,33,37,27,61,86,69,43,13,15,68
+79,77,28,57,26,87,69,19,96,82,18,52,61
+94,52,27,14,61,37,96,69,33,28,87,26,82,19,79,35,77,54,39,97,23
+66,31,24,84,47,68,74
+88,31,49,84,93,59,53,64,86,43,15,18,78,75,94,52,27,14,61
+78,86,93,94,46,61,75,47,64,88,52,27,59,18,31,49,57,15,14,43,84,53,13
+35,99,24,77,52,14,23,79,69,67,33,82,61,26,28
+99,97,31,43,23,55,84
+88,23,43,99,24,84,31,12,68,46,93,55,15,21,47,66,57,64,59,53,74,49,13
+19,79,35,77,67,54,62,97,23,24,99,66,12,21,74,55,88,47,31,93,59
+31,93,59,46,68,43,18,78,94,14,61,37,96
+19,54,62,23,99,12,21,55,49,93,59
+37,26,75,78,86,64,18,94,82,33,68,87,52,43,57
+14,33,87,77,67,54,62,24,12
+79,55,97,54,21,74,28,35,96,26,62,67,82,66,87
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+86,75,94,61,28,35,67
+53,64,78,94,96,28,26
+59,53,64,86,18,27,37,96,87
+77,67,54,62,97,23,99,12,21,74,88,47,84,93,59,53,64
+13,64,46,57,86,43,15,78,75,94,52,27,14,61,37,96,69,33,87
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+99,66,12,21,55,88,47,31,49,84,93,59,53,13,64,46,68,57,86,15,78
+68,57,94,52,27,96,69,33,28
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+12,74,88,47,49,84,53,46,86,18,94
+67,54,62,97,23,12,21,88,31,84,93
+53,43,78,18,84,75,57,52,94,21,47,68,93
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+15,18,78,75,94,52,27,14,61,37,96,69,33,26,82,19,35,77,67,54,39
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+97,12,61,33,26,77,28,24,66,37,39,21,69,54,35
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+67,23,21,74,55
+47,23,26,88,82,28,87,66,67,99,55,21,19,54,79,39,31,24,97,12,74,62,35
+12,31,84,53,57,86,94
+18,52,28,94,93,86,53,68,13,64,27
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+88,47,31,49,84,93,59,13,68,57,86,43,18,78,75,52,27,14,61
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+68,57,78,94,96,26,19,79,35
+96,33,28,87,26,19,79,35,77,54,39,62,97,23,24,99,66,12,21,74,55
+55,64,15,75,59,66,78,74,68,46,47,88,18,12,84
+59,13,15,18,78,94,69,28,87
+86,46,66,12,99,13,47,68,15,93,88,43,59,24,21
+12,47,93,43,21,53,88,55,59,31,13,74,49,94,15,18,46
+97,37,52,67,39,99,69,14,87
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+77,67,54,62,24,12,21,55,49,84,93,13,64
+69,28,87,19,35,77,67,39,62,97,23,24,99,66,74,55,88
+35,77,67,54,39,62,97,23,24,99,12,74,55,88,47,49,84,93,59,53,13
+37,96,69,26,54,62,97,23,24,99,74
+14,93,15,68,13,46,57,27,59,52,64,75,84,43,86,18,47,78,37,49,94,31,61
+93,59,53,13,64,46,68,57,86,43,78,75,94,27,14,61,37,96,69,33,28
+67,84,54,19,99,49,62,77,93,66,47,97,74,79,12,35,23,59,21,31,39
+23,24,74,88,59,64,86
+74,55,88,47,31,49,84,93,59,13,64,46,68,57,86,43,15,18,78,75,94,52,27
+69,26,75,94,86
+93,55,78,47,15,74,52,21,31,64,18
+23,24,99,66,12,21,74,55,88,47,49,84,59,53,13,64,46,68,57,86,15
+97,67,55,96,74,87,82
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+74,55,88,31,49,93,59,13,64,46,68,57,86,43,15,18,78,75,27
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+55,47,31,53,78,75,14
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+87,62,88,35,47,24,28,26,66,33,23,99,67,39,97,79,19,74,77
+31,52,49,78,59,57,21,84,86,18,15
+15,18,78,75,94,52,27,14,61,37,96,69,33,28,87,82,19,79,35,77,67,54,39
+64,93,13,84,49,43,86,47,74,12,68,59,23,21,15,88,57,66,24,99,55
+77,67,54,97,99,12,21,55,88,49,84,59,53,13,64
+35,67,54,97,23,66,12,21,74,55,88,47,31,49,84,93,59,53,13
+43,33,37,77,54,96,18,94,75,52,14
+26,79,67,39,97,23,66,12,21,74,55,88,47,49,84
+62,93,64,86,46,21,47
+67,54,39,62,97,23,24,99,66,74,88,47,49,93,59,53,13,64,46
+31,93,68,75,94,14,96
+94,14,61,96,33,28,87,82,19,79,35,77,67,39,97,23,24
+99,66,12,21,74,55,88,47,31,49,84,93,59,13,64,46,57,86,43,18,78
+37,96,33,82,39
+79,35,77,97,99,66,47,31,53
+49,84,93,53,13,64,46,68,57,86,43,18,78,94,52,27,61,37,69
+18,78,94,14,37,96,33,26,77,54,62
+74,55,88,49,59,13,64,46,86,43,18,75,94,52,27
+93,59,53,46,68,18,78,14,96,33,28
+75,94,61,69,33,28,87,77,67,54,23
+97,47,24,31,68,12,54
+13,46,68,86,43,15,18,75,94,52,27,37,33,28,87,26,82
+35,87,26,33,96,18,78,15,61,54,69,39,67,94,75,77,82
+78,94,52,14,28,82,79,77,54,39,97
+77,62,49,59,53
+87,82,77,67,54,62,66,12,21,74,55,47,49
+68,57,43,78,75,94,52,27,37,33,28,87,26,82,19,79,35
+12,31,84,47,24,66,13,93,67,97,21,35,55
+86,43,15,75,52,37,96,33,28,87,26,82,79,77,67
+14,61,37,96,69,33,87,26,82,67,54,39,62,23,99,66,12
+47,31,49,84,93,59,53,13,64,46,68,57,86,43,15,18,78,75,94,27,14,61,37
+62,24,99,66,12,21,74,55,88,47,31,49,84,93,59,53,13,64,68,57,86
+39,79,69,77,75,82,78,15,18,37,61
+39,21,28,54,77,82,88,74,87
+69,33,28,87,26,82,19,79,35,67,54,39,62,97,23,24,99,66,12,21,74,55,88
+86,43,15,18,78,75,94,52,14,61,96,69,33,28,87,26,82,19,35
+66,12,47,49,84,59,53,13,64,15,75
+46,68,57,86,43,15,18,78,75,94,52,27,14,61,37,96,69,33,28,87,26,82,79
\ No newline at end of file
diff --git a/day_5/solve_1.test.ts b/day_5/solve_1.test.ts
new file mode 100644
index 0000000..b369fef
--- /dev/null
+++ b/day_5/solve_1.test.ts
@@ -0,0 +1,90 @@
+import { assertArrayIncludes, assertEquals } from "jsr:@std/assert";
+import { makeTopologySortOrder, readFormat } from "./solve_1.ts";
+
+Deno.test("readFormat", async () => {
+    const path = new URL("example.txt", import.meta.url).pathname;
+    const actual = await readFormat(path);
+    const expected = {
+        order: [
+            { left: 47, right: 53 },
+            { left: 97, right: 13 },
+            { left: 97, right: 61 },
+            { left: 97, right: 47 },
+            { left: 75, right: 29 },
+            { left: 61, right: 13 },
+            { left: 75, right: 53 },
+            { left: 29, right: 13 },
+            { left: 97, right: 29 },
+            { left: 53, right: 29 },
+            { left: 61, right: 53 },
+            { left: 97, right: 53 },
+            { left: 61, right: 29 },
+            { left: 47, right: 13 },
+            { left: 75, right: 47 },
+            { left: 97, right: 75 },
+            { left: 47, right: 61 },
+            { left: 75, right: 61 },
+            { left: 47, right: 29 },
+            { left: 75, right: 13 },
+            { left: 53, right: 13 },
+        ],
+        pageNumbers: [
+            [75, 47, 61, 53, 29],
+            [97, 61, 53, 29, 13],
+            [75, 29, 13],
+            [75, 97, 47, 61, 53],
+            [61, 13, 29],
+            [97, 13, 75, 29, 47],
+        ],
+    };
+    assertEquals(actual, expected);
+});
+
+Deno.test("makeTopologySortOrder", () => {
+    const orderMap = new Map<number, Set<number>>([
+        [1, new Set([2, 3])],
+        [2, new Set([4])],
+        [3, new Set([4])],
+        [4, new Set()],
+    ]);
+    const actual = makeTopologySortOrder(orderMap);
+    const expected = [[4, 2, 3, 1], [4, 3, 2, 1]];
+    // 4 -> 2 -> 3 -> 1
+    // 4 -> 3 -> 2 -> 1
+    // multiple valid topological sort order
+    assertArrayIncludes(expected, [actual]);
+});
+
+Deno.test("makeTopologySortOrder with cycle", () => {
+    const orderMap = new Map<number, Set<number>>([
+        [1, new Set([2])],
+        [2, new Set([3])],
+        [3, new Set([1])],
+    ]);
+    const actual = makeTopologySortOrder(orderMap);
+    const expected = [[3, 2, 1], [2, 1, 3], [1, 3, 2]]; // or any valid topological sort order
+    // 3 -> 1 -> 2 -> 3
+    // 2 -> 3 -> 1 -> 2
+    // 1 -> 2 -> 3 -> 1
+    // multiple valid topological sort order
+
+    // The order of the nodes in the cycle is not guaranteed.
+    assertArrayIncludes(expected, [actual]);
+});
+
+Deno.test("makeTopologySortOrder with disconnected graph", () => {
+    const orderMap = new Map<number, Set<number>>([
+        [1, new Set([2])],
+        [3, new Set([4])],
+    ]);
+    const actual = makeTopologySortOrder(orderMap);
+    const expected = [
+        [2, 1, 4, 3],
+        [2, 4, 1, 3],
+        [2, 4, 3, 1],
+        [4, 2, 1, 3],
+        [4, 2, 3, 1],
+        [4, 3, 2, 1],
+    ];
+    assertArrayIncludes(expected, [actual]);
+});
diff --git a/day_5/solve_1.ts b/day_5/solve_1.ts
new file mode 100644
index 0000000..0a48ad8
--- /dev/null
+++ b/day_5/solve_1.ts
@@ -0,0 +1,108 @@
+export async function readFormat(path: string) {
+    const data = await Deno.readTextFile(path);
+    // assume LF line endings.
+    // TODO: handle CRLF line endings.
+    const [orderTxt, pageNumbersTxt] = data.split("\n\n");
+
+    const order = orderTxt.split("\n").map((line) => {
+        const [left, right] = line.split("|");
+        return {
+            left: parseInt(left.trim()),
+            right: parseInt(right.trim()),
+        };
+    });
+    const pageNumbers = pageNumbersTxt.split("\n").map((line) => {
+        const numbers = line.split(",");
+        return numbers.map((num) => parseInt(num.trim()));
+    });
+    return { order, pageNumbers };
+}
+
+// topology sort order
+export function makeTopologySortOrder(
+    orderMap: Map<number, Set<number>>,
+): number[] {
+    const visited = new Set<number>();
+    const sorted = new Array<number>();
+    for (const key of orderMap.keys()) {
+        if (!visited.has(key)) {
+            visit(key);
+        }
+    }
+    function visit(node: number) {
+        if (visited.has(node)) return;
+        visited.add(node);
+        const set = orderMap.get(node);
+        if (set) {
+            for (const next of set) {
+                visit(next);
+            }
+        }
+        sorted.push(node);
+    }
+    return sorted;
+}
+
+async function main() {
+    const path = new URL("input.txt", import.meta.url).pathname;
+    const { order, pageNumbers } = await readFormat(path);
+
+    // create order map
+    const orderMap = new Map<number, Set<number>>();
+    for (const { left, right } of order) {
+        const leftSet = orderMap.get(left) ?? new Set<number>();
+        leftSet.add(right);
+        orderMap.set(left, leftSet);
+    }
+    // it's not transitive closure...
+    //
+    // apply transitive closure
+    // const topologySortOrder = makeTopologySortOrder(orderMap);
+    // const completedSetMap = new Map<number, Set<number>>();
+    // for (const num of topologySortOrder) {
+    //     // console.log(num, "\n===");
+    //     const set = orderMap.get(num) ?? new Set<number>();
+    //     const completedSet = completedSetMap.get(num) ?? new Set<number>();
+    //     set.forEach((elem) => completedSet.add(elem));
+    //     for (const next of set) {
+    //         const nextSet = completedSetMap.get(next) ?? new Set<number>();
+    //         nextSet.forEach((elem) => completedSet.add(elem));
+    //     }
+    //     console.log(completedSet);
+    //     // console.log("===");
+    //     completedSetMap.set(num, completedSet);
+    // }
+
+    const completedSetMap = orderMap;
+
+    // check order is valid or not by comparing with pageNumbers
+    function checkPageNumberOrder(pageNumbers: number[]): boolean {
+        const notAllowed = new Set<number>();
+        for (let i = pageNumbers.length - 1; i >= 0; i--) {
+            const current = pageNumbers[i];
+            if (notAllowed.has(current)) {
+                return false;
+            }
+            notAllowed.add(current);
+            const biggerThanCurrent = completedSetMap.get(current);
+            biggerThanCurrent?.forEach((elem) => notAllowed.add(elem));
+        }
+        return true;
+    }
+
+    let sum = 0;
+    for (const pages of pageNumbers) {
+        if (!checkPageNumberOrder(pages)) {
+            // console.log("invalid order", pages);
+        } else {
+            // get middle number
+            const middle = pages[pages.length >> 1];
+            sum += middle;
+        }
+    }
+    console.log(sum);
+}
+
+if (import.meta.main) {
+    main();
+}
diff --git a/day_5/solve_2.ts b/day_5/solve_2.ts
new file mode 100644
index 0000000..49b53c5
--- /dev/null
+++ b/day_5/solve_2.ts
@@ -0,0 +1,86 @@
+import { readFormat } from "./solve_1.ts";
+
+async function main() {
+    const path = new URL("input.txt", import.meta.url).pathname;
+    const { order, pageNumbers } = await readFormat(path);
+
+    // create order map
+    const orderMap = new Map<number, Set<number>>();
+    for (const { left, right } of order) {
+        const leftSet = orderMap.get(left) ?? new Set<number>();
+        leftSet.add(right);
+        orderMap.set(left, leftSet);
+    }
+    const completedSetMap = orderMap;
+    
+    // check order is valid or not by comparing with pageNumbers
+    function checkPageNumberOrder(pageNumbers: number[]): boolean {
+        const notAllowed = new Set<number>();
+        for (let i = pageNumbers.length - 1; i >= 0; i--) {
+            const current = pageNumbers[i];
+            if (notAllowed.has(current)) {
+                // console.log("not allowed", current, notAllowed  );
+                return false;
+            }
+            notAllowed.add(current);
+            const biggerThanCurrent = completedSetMap.get(current);
+            biggerThanCurrent?.forEach((elem) => notAllowed.add(elem));
+        }
+        return true;
+    }
+
+    function findErrorIndex(n: number, pages: number[]) {
+        return pages.findIndex(page => {
+            const biggerThanCurrent = completedSetMap.get(page);
+            if (biggerThanCurrent) {
+                return biggerThanCurrent.has(n);
+            }
+            return false;
+        })
+    }
+    
+    function correctPageNumberOrder(pageNumbers: number[]): number[] {
+        const result = [...pageNumbers];
+        const notAllowed = new Set<number>();
+        for (let i = result.length - 1; i >= 0; i--) {
+            const current = result[i];
+            if (notAllowed.has(current)) {
+                const p = result.slice(i + 1);
+                const errorIndex = findErrorIndex(current, p);
+                if (errorIndex === undefined) {
+                    // could not happen
+                    throw new Error("cannot correct the order");
+                }
+                const ei = errorIndex + i + 1;
+                result[i] = result[ei];
+                result[ei] = current;
+
+                return correctPageNumberOrder(result);
+            }
+            notAllowed.add(current);
+            const biggerThanCurrent = completedSetMap.get(current);
+            biggerThanCurrent?.forEach((elem) => notAllowed.add(elem));
+        }
+        return result;
+    }
+
+    let sum = 0;
+    for (const pages of pageNumbers) {
+        if (!checkPageNumberOrder(pages)) {
+            // correct the order
+            const corrected = correctPageNumberOrder(pages);
+            if (!checkPageNumberOrder(corrected)) {
+                console.log("cannot correct the order", pages);
+                console.log("corrected", corrected);
+                throw new Error("cannot correct the order");
+            }
+            const middle = corrected[corrected.length >> 1];
+            sum += middle;
+        }
+    }
+    console.log(sum);
+}
+
+if (import.meta.main) {
+    main();
+}