how to pass object to a function in inheritance or polymorph

Question

 hello, am using this code
pls give me some response it is correct or wrong

using System;

class first

{

    protected int num1, num2;

    public first(int a = 0, int b = 0)

    {

        num1 = a;

        num2 = b;

    }

    public virtual int sum()

    {

        Console.WriteLine("sum is :");

        return 0;

    }

}

class second : first

{

    public second(int a = 0, int b = 0)

        : base(a, b)

    {

    }

    public override int sum()

    {

        Console.WriteLine("sum is :");

        return (num1 + num2);

    }

}

class three : first

{

    public three(int a = 0, int b = 0)

        : base(a, b)

    {

    }

    public override int sum()

    {

        Console.WriteLine("value is :");

        return (num1 + num2);

    }

}

class Call

{

    public void callsum(first fr)

    {

        int a;

        a = fr.sum();

        Console.WriteLine("sum: {0}", a);

    }

}

class Test

{

    static void Main(string[] args)

    {

        Call c = new Call();

        second s = new second(10, 7);

        three t = new three(10, 5);

        c.callsum(s);

        c.callsum(t);

        Console.ReadKey();

    }

}