public class D
{
    public static void main(String[] args){
        System.out.println("Test: calcSum");
        int n = 0;
        if (n>=0) System.out.println("n: " + n + ", sum: " + calcSum(n));
        else System.out.println("n: " + n + ", sum: error n<0!");
        }

        static int calcSum(int n){
            if (n==0) return 1;
            int sum = n;
            for(int i = n-1; i >= 1; i--) {
                sum = sum * i;
            }
            return sum;
            }
}