// editor3
#include<stdio.h>
long long fact(int);
int main()
{
    int num, factorial;
    scanf("%d",&num);
    
    if(num<0)
    {
        printf("Invalid input");
        return 0;
    }
    
    if(num<=20)
    {
        long long int f = fact(num);
        printf("%lld",f);
    }
    return 0;
}

unsigned long long fact(int num)
{
    if (num==0 || num==1)
        return 1;
    
    return num * fact(num-1);
    //return f;
    //return 0;
}