diff --git a/2021/14.js b/2021/14.js
new file mode 100644
index 0000000..e220c1b
--- /dev/null
+++ b/2021/14.js
@@ -0,0 +1,37 @@
+function day14(input, part2) {
+    input = input.split("\n").splitOnElement("")
+
+    let init = input[0][0].split("")
+    let rules = input[1].map((e) => e.split(" -> "))
+
+    let counts = {}
+
+    for (let i = 0; i < init.length - 1; i++) {
+        let substr = init[i] + init[i + 1]
+        counts[substr] = (counts[substr] || 0) + 1
+    }
+
+    for (let i = 0; i < (part2 ? 40 : 10); i++) {
+        let changes = {}
+
+        for (let rule of rules) {
+            let target = rule[0][0] + rule[0][1]
+            let left = rule[0][0] + rule[1]
+            let right = rule[1] + rule[0][1]
+
+            changes[left] = (changes[left] || 0) + (counts[target] || 0)
+            changes[right] = (changes[right] || 0) + (counts[target] || 0)
+            changes[target] = (changes[target] || 0) - (counts[target] || 0)
+        }
+
+        for (let pair in changes) {
+            counts[pair] = (counts[pair] || 0) + changes[pair]
+        }
+    }
+
+    counts = Object.keys(counts).map((key) => [key.split(""), counts[key]])
+
+    let letters = counts.map((e) => e[0]).flat().uniq().map((letter) => (counts.filter((e) => e[0].includes(letter)).map((e) => e[0].count(letter) * e[1]).sum() + (letter == init[0] || letter == init[init.length - 1])) / 2)
+
+    return letters.max() - letters.min()
+}