diff --git a/15/15.py b/15/15.py
new file mode 100644
index 0000000..ed0ff29
--- /dev/null
+++ b/15/15.py
@@ -0,0 +1,22 @@
+from collections import defaultdict
+
+numbers = [14,1,17,0,3,20]
+
+def f(numbers, nth):
+    d={}
+    for i, n in enumerate(numbers[:-1], 1):
+        d[n] = i
+
+    prev = numbers[-1]
+    for i in range(len(numbers), nth):
+        if prev not in d:
+            d[prev] = i
+            prev = 0
+        else:
+            next = i-d[prev]
+            d[prev] = i
+            prev = next
+    return prev
+
+print(f(numbers, 2020))
+print(f(numbers, 30_000_000))