Fix: order of operands in subtraction, error msgs

Improved error messages for integer overflows and division errors

main.cpp

@@ -13,7 +13,7 @@ int add(int a, int b) {
   int res;
   bool overflow = __builtin_sadd_overflow(a, b, &res);
   if (overflow) {
-    throw std::invalid_argument("Malformed input. Integer overflow.");
+    throw std::invalid_argument("Malformed input. Integer overflow (add).");
   }
   return res;
 }
@@ -35,9 +35,9 @@ int multiply(int a, int b) {
  */
 int subtract(int a, int b) {
   int res;
-  bool overflow = __builtin_ssub_overflow(a, b, &res);
+  bool overflow = __builtin_ssub_overflow(b, a, &res);
   if (overflow) {
-    throw std::invalid_argument("Malformed input. Integer overflow.");
+    throw std::invalid_argument("Malformed input. Integer overflow (subtract).");
   }
   return res;
 }
@@ -48,7 +48,7 @@ int subtract(int a, int b) {
  */
 int divide(int a, int b) {
   if (b == 0) {
-    throw std::invalid_argument("Tried to divide by 0.");
+    throw std::invalid_argument("Malformed input. Division by zero.");
   }
   return a / b;
 }