Abstraction with C#

abstract class AreaClass
    {
        abstract public void Area();
    }


    class Square : AreaClass
    {
        int a;
        public Square(int a)
        {
            this.a = a;
        }
        public override void Area()
        {
            Console.WriteLine(a * a);
        }
    }
    internal class Program
    {
        static void Main(string[] args)
        {
            Square s = new Square(4);
            s.Area();
            Console.ReadKey();
        }

    }