#include <stdio.h>

unsigned long long factorial(int n) {
    if (n == 0 || n == 1) return 1;
    unsigned long long result = 1;
    for (int i = 2; i <= n; i++) {
        result *= i;
    }
    return result;
}

int main() {
    double input;
    scanf("%lf", &input);

    if (input < 0 || (input - (int)input) != 0) {
        printf("Invalid input\n");
    } else {
        int n = (int)input;
        unsigned long long result = factorial(n);
        printf(%llu\n",result);
    }
    return 0;
}