#include <iostream>
using namespace std;

int fact(int);

void main()
{
    int n;
    cout << "This program calculates  n! . \n"
         << "n = ";
    cin >> n;

    cout << fact(n) << endl;
}

int fact(int b)
{
    if (b < 0)
        return 1;
    else
        return b * fact(b - 1);
}