[POH7]彼女に水着をプレゼント(C++)

#include <iostream>
using namespace std;

int main(void){
　size_t billion = 1000000000;
  string str;
  getline(cin, str);
  size_t i = stoi(str), num = 1;
  for (size_t j = i; j > 0; --j)
    {
        num = num * j;
        while(num % 10 == 0){
            num = num / 10;
        }
        if(num > billion)
            num = num % (billion*10);
    }
    num = num % billion;
    cout << num << endl;
  return 0;
}