implemented subtraction

rpn.py

@@ -9,7 +9,10 @@ def calculate(arg):
         except ValueError:
             val1 = stack.pop()
             val2 = stack.pop()
-            result = val1 + val2
+            if token == '+':
+                result = val1 + val2
+            elif token == '-':
+                result = val2 - val1
             stack.append(result)
     return stack[0]