#include <cmath>
#include <fstream>
#include <iostream>
using namespace std;

int calcFact(int n) {
    if (n == 0) return 1;
    return n * calcFact(n - 1);
}

int main() {
    int n, x;
    ifstream f("fact.in");
    ofstream g("fact.out");
    f >> n;

    if (n < 0)
        g << "-1" << endl;
    else {
        g << calcFact(n) << endl;
    }

    f.close();
    g.close();
}