diff --git a/2020/d21/ex1/ex1.py b/2020/d21/ex1/ex1.py
new file mode 100755
index 0000000..f305249
--- /dev/null
+++ b/2020/d21/ex1/ex1.py
@@ -0,0 +1,64 @@
+#!/usr/bin/env python
+
+import functools
+import re
+import sys
+from collections import defaultdict
+from copy import deepcopy
+from typing import Dict, List, Set, Tuple
+
+
+def parse(raw: List[str]) -> Tuple[List[Set[str]], List[Set[str]]]:
+    def parse_ingredients(line: str) -> Set[str]:
+        pos = line.find(" (contains ")
+        if pos != -1:
+            line = line[:pos]
+        return set(line.split())
+
+    def parse_allergens(line: str) -> Set[str]:
+        pos = line.find("(contains ")
+        if pos == -1:
+            return set()
+        return set(re.findall("([^ ]+)[,\\)]", line))
+
+    ingredients = []
+    allergens = []
+
+    for line in raw:
+        ingredients.append(parse_ingredients(line))
+        allergens.append(parse_allergens(line))
+
+    return ingredients, allergens
+
+
+def find_allergens(
+    ingredients: List[Set[str]], allergens: List[Set[str]]
+) -> Dict[str, Set[str]]:
+    all_ingredients = functools.reduce(lambda lhs, rhs: lhs | rhs, ingredients)
+    possibilities: Dict[str, Set[str]] = defaultdict(lambda: deepcopy(all_ingredients))
+
+    for ing, all in zip(ingredients, allergens):
+        for allergen in all:
+            possibilities[allergen] &= set(ing)
+
+    return dict(possibilities)
+
+
+def solve(raw: List[str]) -> int:
+    ingredients, allergens = parse(raw)
+    possibilities = functools.reduce(
+        lambda lhs, rhs: lhs | rhs, find_allergens(ingredients, allergens).values()
+    )
+
+    return sum(
+        bool(ingredient not in possibilities) for i in ingredients for ingredient in i
+    )
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()