using System;

public class Program

{

    public static void Main()

    {

         long result = Sum(10, num2nd: 20,30);//this will throw error in any c# version less than 7.2

​

            Console.WriteLine("result : "+result);

            Console.ReadLine();

    }

     public static long Sum(int num1st, int num2nd=default, int num3rd = default)

        {

            return num1st + num2nd + num3rd;

        }

}