Abstract class using c#

using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication31
{
abstract class a
{
public abstract void add();

public void sub()
{
Console.WriteLine("base class a");
}
}
class b:a
{

public override void add()
{
Console.WriteLine("derived class b");
}
}
class c
{
static void Main(string[] args)
{
b b1 = new b();
b1.add();
b1.sub ();
}
}

}