using System;

using System.Collections.Generic;

using System.Linq;

​

interface IApp

     {

       int Sum(int i, int j); //By Default Public

     }

​

public class Program : IApp

{

    public int Sum(int i, int j) //Must be declare Public

        {

           return i + j;

        }

    public static void Main()

    {

        Program obj = new Program();

        Console.WriteLine(obj.Sum(2, 3));

    }

}