#include<stdio.h>
long int factorial(int n){
    if (n == 0){//base case
        return 1;
    }
    else{
        return n * factorial(n-1);//recursive call
    }
}
int main(){
    int num;
    scanf("%d",&num);
    if(num<0){
        printf("Invalid input");
        return 0;
    }
    long int result = factorial(num);
    printf("Invalid input";)
    return 0;
}