public  class Factorial2 {
    public static void main(String[] args) {
        System.out.println(factorial(-1));
    }

    public static int factorial(int n) {
        if (n < 0)
            throw new IllegalArgumentException("negative n");
        int product = 1;
        for (int i = 2; i <= n; i++)
            product *= i;
        return product;
    }
}