Parse zero-padded number

zero_padded.cpp

#include <iostream>
#include <string>
#include <stdexcept>

int main(int, char *[])
{
  using std::string;
  using std::cin;
  using std::cout;
  using std::endl;
  using std::runtime_error;

  string input_num;
  int converted_num = 0;

  cout << "Enter 4 char number..." << endl;
  cin >> input_num;

  try {
    if(input_num.size() != 4) {
      throw runtime_error("Input must be 4 characters long!");
    }

    for(auto &&c : input_num) {
      auto i = c - '0';
      if(i < 0 || i > 9) {
        throw runtime_error("Input must contain numbers only!");
      }

      converted_num = (converted_num * 10) + i;
    }

    cout << "Result: " << converted_num << endl;
  }
  catch(runtime_error &e) {
    cout << e.what() << endl;
  }

  return 0;
}