#include <iostream>
using namespace std;
int main() {
    int n, i;
    unsigned long long fact = 1;
    cout << "Enter an integer: ";
    cin >> n;
    // shows error if the user enters a negative integer
    if (n < 0)
        cout << "Factorial of a negative number doesn't exist.";
    else {
        for (i = 1; i <= n; ++i) {
            fact *= i;
        }
        cout << "Factorial = " << fact;
    }
    return 0;
}