diff --git a/2021/day3.py b/2021/day3.py
new file mode 100644
index 0000000..0c763d2
--- /dev/null
+++ b/2021/day3.py
@@ -0,0 +1,32 @@
+import sys
+
+
+def main():
+    lines = [line.rstrip() for line in sys.stdin]
+
+    # Part 1
+    cumul = [0] * len(lines[0])
+    for line in lines:
+        for i, b in enumerate(line):
+            cumul[i] += 1 if b == "1" else -1
+    gamma = int("".join(map(lambda n: "1" if n > 0 else "0", cumul)), 2)
+    epsilon = gamma ^ (2 ** len(cumul) - 1)  # why does ~ not work? ugly mask
+    print(gamma * epsilon)
+
+    # Part 2
+    ogr_values = lines[:]
+    csr_values = lines[:]
+    for i in range(len(cumul)):
+        if len(ogr_values) > 1:
+            cumul = sum(1 if v[i] == "1" else -1 for v in ogr_values)
+            ogr_mc = 1 if cumul >= 0 else 0
+            ogr_values = [v for v in ogr_values if int(v[i]) == ogr_mc]
+        if len(csr_values) > 1:
+            cumul = sum(1 if v[i] == "1" else -1 for v in csr_values)
+            csr_lc = 0 if cumul >= 0 else 1
+            csr_values = [v for v in csr_values if int(v[i]) == csr_lc]
+    print(int(ogr_values[0], 2) * int(csr_values[0], 2))
+
+
+if __name__ == "__main__":
+    main()