Abstract class using adding and multiple

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

public abstract   class AddMul
{
    public AddMul()
    {
        //
        // TODO: Add constructor logic here
        //
    }
    private int num1;
    private int num2;
    public int Num1
    {
        get { return num1; }
        set { num1 = value; }
    }
    public int Num2
    {
        get { return num2; }
        set { num2 = value; }
    }
   public abstract  class Class1
    {
        public abstract int AddTwoNumbers(int Num1, int Num2);
        public abstract int MultiplyTwoNumbers(int Num1, int Num2);
    }
    abstract class Class2 : Class1
    {

        public override int AddTwoNumbers(int Num1, int Num2)
        {
            return Num1 + Num2;
        }
    }

    class absDerived : Class2
    {

        public override int MultiplyTwoNumbers(int Num1, int Num2)
        {
            return Num1 * Num2;
        }
    }
}

List<AddMul> admu = new List<AddMul>();
        AddMul am = new AddMul();
        am.Num1 =int.Parse ( txtfirst.Text);
        am.Num2 = int.Parse(txtsecond.Text);