diff --git a/2020/d09/ex1/ex1.py b/2020/d09/ex1/ex1.py
new file mode 100755
index 0000000..318e38f
--- /dev/null
+++ b/2020/d09/ex1/ex1.py
@@ -0,0 +1,32 @@
+#!/usr/bin/env python
+
+
+import itertools
+import sys
+from typing import List
+
+
+def find_invalid(nums: List[int]) -> int:
+    for i in range(25, len(nums)):
+        num = nums[i]
+        found = False
+        for lhs, rhs in itertools.combinations(nums[i - 25 : i], 2):
+            if lhs + rhs == num:
+                found = True
+                break
+        if not found:
+            return num
+    assert False  # Sanity check
+
+
+def solve(raw: List[str]) -> int:
+    return find_invalid([int(line) for line in raw])
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()