diff --git a/buggy.py b/buggy.py
--- a/buggy.py
+++ b/buggy.py
@@ -12,7 +12,7 @@
         stack = []
         for i in operations:
             if i == "D":
-                stack.append(2 * math.notify(stack[-1]))
+                stack.append(2 * stack[-1])
             elif i == "C":
                 stack.pop()
             elif i == "+":
