diff --git a/day21/sol.py b/day21/sol.py
new file mode 100644
index 0000000..6e5ba8e
--- /dev/null
+++ b/day21/sol.py
@@ -0,0 +1,39 @@
+import operator
+
+lines = []
+for line in open("input"):
+    lines.append(line.split())
+
+
+unused = lines
+scope = {}
+while unused:
+    lines, unused = unused, []
+    for words in lines:
+        refs = [x for x in words[1:] if x.isalpha()]
+        if any(r not in scope for r in refs):
+            unused.append(words)
+            continue
+        dst = words[0].rstrip(':')
+        if len(words) == 2:
+            _, src = words
+            scope[dst] = int(src)
+            continue
+        _, x, op, y = words
+        scope[dst] = (op, scope[x], scope[y])
+
+optab = {
+    '*': operator.mul,
+    '/': operator.floordiv,
+    '+': operator.add,
+    '-': operator.sub,
+}
+
+def eval(x):
+    if type(x) == int:
+        return x
+    op = x[0]
+    args = [eval(a) for a in x[1:]]
+    return optab[op](*args)
+
+print(eval(scope['root']))