#include<stdio.h>

int factorial(int n){
    if(n == 0 || n == 1)
    return 1;
  else
     return n * factorial(n - 1);
}

int main(){
    int n;
    scanf("%d",&n);
    
    if(n<0){
        printf("invalid input\n");
    }
    else{
        printf("%d\n",factorial(n));
    }
    
    return 0
}