下面程序运行后输入4，输出结果应是

#include <iostream>
using namespace std;
int fac (int n)
{
  if (n <=1)
        return 1;
    else
        return n * fac (n - 1);
}
int main ()
{
    int n = 0;
    cin >> n;
    int ret = fac (n);
    cout << ret;
    return 0;
}