#include <stdio.h>
long long factorial (int n) {
    if (n == 0 || n == 1)
        return 1;
    else 
        return n * factorial (n - 1);
}
int main() {
    printf("Enter an integer: ");
    scanf("%d, &n");
    if (n < 0) {
        printf("Invalid Input\n");
    } else {
        printf("%lld\n", factorial(n));
    }
    return 0;
}