#include <iostream>
using namespace std;

double fact (int);

int main ()
{
  int i;

  for (i = -10; i <= 100; i+=5)
  {
    cout << i << " factorial is "
         << fact(i) << "."
         << endl;
  }
  return 0;
}

double fact (int num)
{
  double result = 1;
  int i;
 
  if (num < 0)
    result = -1;
  else
    for (i = 2; i <= num; i++)
      result = result * i;

  return result;
}