refactoring: case(for add and subtract)

src/main/c/Calculator/calculator.c

@@ -72,6 +72,10 @@ double combination(int n, int r);
     }
     double performOperation(int choice, double num1, double num2) {
     switch (choice) {
+        case 1:
+            return num1 + num2;
+        case 2:
+            return num1 - num2;