using System;

​

public class Program

{

    public static void Main()

    {

        int n = 7;

        int result = even(n);

        Console.WriteLine(even(7));

    }

​

    public static int even(int n)

    {

​

        if (n < 2)

            return 0;

​

​

        if (n % 2 == 0)

            return n + even(n - 2);

​

​

        return even(n - 1);

    }

}