// editor3
#include<stdio.h>

fact(int);
int main()
{
    int num, factorial;
    scanf("%d",&num);
    
    if(num<0)
    {
        printf("Invalid Input");
        return 0;
    }
    
    if(num<=20)
    {
        factorial = fact(int num);
        return 0;
    }
}

int fact(int num)
{
    int factorial;
    factorial = num * fact(num-1);
    return factorial;
}