#include <functional>
#include <string>
#include <array>
using namespace std;

template <typename T>
string calculator(int op, T x, T y) {
  static array<std::function<T(T, T)>,5> ops{
    plus<>{},minus<>{},multiplies<>{},divides<>{},modulus<>{},
  };
  static array<std::function<string()>, 2> func{
    [&]() {return to_string(ops[op-1](x, y));},
    [&]() {return "Invalid Input!";},
  };

  return (op < 1 || op > 5 || (!y && (op == 4 || op == 5))) ? func[1]() : func[0]();
}