diff --git a/src/calc.py b/src/calc.py
index 2af3c17..bcc717a 100644
--- a/src/calc.py
+++ b/src/calc.py
@@ -9,7 +9,7 @@ def add(a, b, third_operand = 0):
     """
     return a + b + third_operand
 
-def sub(a, b):
+def sub(a, b, c=0):
     """
     Subtract some numbers
 
@@ -17,4 +17,4 @@ def sub(a, b):
     sub(8, 3) # 5
     ```
     """
-    return a - b
+    return a - b - c