using System;

public class Program

{       

    public static void Main()

    {   

​

        int result = AddNumber(20);

        Console.WriteLine($"_ {result} _ should equal 30 so firstNumber is set");

        result = AddNumber(secondNumber: 20);

        Console.WriteLine($"_ {result} _ should equal 20 when secondNumber is explicitly set");

    }

    public static int AddNumber(int firstNumber = 0, int secondNumber = 10)

    {

        return firstNumber+ secondNumber;

    }

}