using System;

​

public class Program

{

    public static void Main()

    {

        int[] array = { 10, 10, 10, 10 };

        int sum = CalculateSum(array);

        Console.WriteLine(sum);

    }

​

    private static int CalculateSum(int[] array, int index = 0)

    {

        if (index >= 0 && index < array.Length)

        {

            return array[index] + CalculateSum(array, index + 1);

        }

​

        return 0;

    }

}