diff --git a/index.ts b/index.ts
index e809564..3ffc6b4 100644
--- a/index.ts
+++ b/index.ts
@@ -1,3 +1,3 @@
-import {runDayEight} from "./src/day_eight";
+import {runDayNine} from "./src/day_nine";
 
-runDayEight();
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+runDayNine();
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diff --git a/src/day_nine.test.ts b/src/day_nine.test.ts
new file mode 100644
index 0000000..a9e7dc7
--- /dev/null
+++ b/src/day_nine.test.ts
@@ -0,0 +1,35 @@
+import {Oasis} from "./day_nine";
+
+describe('Day Nine', () => {
+    const input = `0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45`;
+
+    it.each([
+        [[0, 3, 6, 9, 12, 15], 18],
+        [[1, 3, 6, 10, 15, 21], 28],
+        [[10, 13, 16, 21, 30, 45], 68]
+    ])('It should get the next value in the sequence', (sequence, expected) => {
+        const oasis = new Oasis(input);
+        expect(oasis.getNextValueInSequence(sequence)).toEqual(expected);
+    });
+
+    it('should get the total of the next values in each sequence', () => {
+        const oasis = new Oasis(input);
+        expect(oasis.getTotalOfNextValuesInSequences()).toEqual(114);
+    });
+
+    it.each([
+        [[0, 3, 6, 9, 12, 15], -3],
+        [[1, 3, 6, 10, 15, 21], 0],
+        [[10, 13, 16, 21, 30, 45], 5]
+    ])('It should get the previous value in the sequence', (sequence, expected) => {
+        const oasis = new Oasis(input);
+        expect(oasis.getPreviousValueInSequence(sequence)).toEqual(expected);
+    });
+
+    it('should get the total of the previous values in each sequence', () => {
+        const oasis = new Oasis(input);
+        expect(oasis.getTotalOfPreviousValuesInSequences()).toEqual(2);
+    });
+});
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diff --git a/src/day_nine.ts b/src/day_nine.ts
new file mode 100644
index 0000000..7553bba
--- /dev/null
+++ b/src/day_nine.ts
@@ -0,0 +1,44 @@
+import fs from "fs";
+
+export class Oasis {
+    private readonly sequences: number[][];
+
+    constructor(input: string) {
+        this.sequences = input.split('\n').map(line => line.split(' ').map(Number));
+    }
+
+    public getNextValueInSequence(seq: number[]): number {
+        if (new Set(seq).size === 1) return seq[0];
+        const differences = seq.reduce((acc: number[], curr, i, arr) => {
+            if (i === 0) return acc;
+            acc.push(curr - arr[i - 1]);
+            return acc;
+        }, []);
+        return this.getNextValueInSequence(differences) + seq[seq.length - 1];
+    }
+
+    public getPreviousValueInSequence(seq: number[]): number {
+        if (new Set(seq).size === 1) return seq[0];
+        const differences = seq.reduce((acc: number[], curr, i, arr) => {
+            if (i === 0) return acc;
+            acc.push(curr - arr[i - 1]);
+            return acc;
+        }, []);
+        return seq[0] - this.getPreviousValueInSequence(differences);
+    }
+
+    public getTotalOfNextValuesInSequences(): number {
+        return this.sequences.reduce((acc, curr) => acc + this.getNextValueInSequence(curr), 0);
+    }
+
+    public getTotalOfPreviousValuesInSequences(): number {
+        return this.sequences.reduce((acc, curr) => acc + this.getPreviousValueInSequence(curr), 0);
+    }
+}
+
+export const runDayNine = () => {
+    const input = fs.readFileSync('./inputs/day_nine_input.txt', 'utf-8').trimEnd();
+    const oasis = new Oasis(input);
+    console.log(oasis.getTotalOfNextValuesInSequences());
+    console.log(oasis.getTotalOfPreviousValuesInSequences());
+}
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