using System;

abstract class periodical {

    public abstract void prelim();

    public abstract void midterms();

}

public interface period {

    public void prefinals();

    public void finals();

}

class semester1: periodical {

    public override void prelim() {

        Console.WriteLine("Prelims");   

    }

    public override void midterms() {

        Console.WriteLine("Midterms");  

    }

}

​

class semester2: period{

    public void prefinals() {

        Console.WriteLine("Pre-finals");    

    }

    public void finals() {

        Console.WriteLine("Finals");    

    }

}

​

public class Program

{

    public static void Main()

    {

        periodical a = new semester1();

        a.prelim();

        a.midterms();

        period b = new semester2();

        b.prefinals();

        b.finals();

    }

}