diff --git a/perimeter-of-squares-in-a-rectangle/index.js b/perimeter-of-squares-in-a-rectangle/index.js
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+// https://www.codewars.com/kata/perimeter-of-squares-in-a-rectangle/javascript
+
+const perimeter = (() => {
+  const map = new WeakMap();
+  const f = n => {
+    if (n in map) return map[n];
+    if (n === 0) return fib(n) * 4;
+    return map[n] = 4 * fib(n) + f(n-1);
+  };
+  return f;
+})();
+
+// https://gist.github.com/gkucmierz/9bc8126bdb8d42a17ccb25f81bf02519
+const fib = ((fst, snd) => {
+  const map = new WeakMap();
+  const f = n => {
+    if (n in map) return map[n];
+    if (n === 0) return fst;
+    if (n === 1) return snd;
+    return map[n] = f(n-1) + f(n-2);
+  };
+  return f;
+})(1, 1);