#include <iostream>
using namespace std;
int fact( int num );

int main() {
   int num;
   cout << "Please enter a number: ";
   cin >> num;
   if ( num < 0 )
      cout << "ERROR--number must be >= 0." << endl;
   else
      cout << num << "! = " << fact( num ) << endl;
   return 0;
}

int fact( int num ) {
   if ( num <= 1 )  // base cases
      return 1;
   // recursive call
   return num * fact( num - 1 );
}