using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

}

​

abstract class A

{

    public abstract void M();

}

​

abstract class B : A { }

​

class C : B

{

    override public void M(){}

}

​

abstract class D : C

{

     public override abstract void M();

}

​