Fixed division by 0 fixed.

src/calc.cpp

@@ -6,7 +6,12 @@
 int plus(    int a, int b) { return a + b; }
 int minus(   int a, int b) { return a - b; }
 int multiply(int a, int b) { return a * b; }
-int divide(  int a, int b) { return a / b; }
+int divide(  int a, int b) { 
+  if(b == 0) {
+    throw std::invalid_argument("Division by 0.");
+  }
+  return a / b;
+}
 
 int pop_stack(auto &stack) {
   const auto ret{stack.top()};