add a subtraction operation

Add member-function which performs a subtraction in terms of
an addition.

Signed-off-by: Sergii Piatakov <sergii.piatakov@globallogic.com>

calculator.cpp

@@ -4,3 +4,8 @@ int Calculator::Add (int a, int b)
 {
     return a + b;
 }
+
+int Calculator::Sub (int a, int b)
+{
+    return Add (a, -b);
+}

calculator.h

@@ -5,6 +5,7 @@ class Calculator
 {
     public:
         int Add (int, int);
+        int Sub (int, int);
 };
 
 #endif//CALCULATOR_H