Double Factorial

import java.util.Scanner;


public class DoubleFactorial {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        System.out.println(dblFact(n));

    }

    private static long dblFact(int n) {
        if (n < 0) {
            return -1;
        }
        if (n == 0 || n == 1) {
            return 1;
        }
        return n * dblFact(n - 2);
    }

}