using System;

public class Program

{

    class Document {

        int number {get; set;}

        double sum {get; set;}

    }

​

    class Bill : Document {

        double advance {get; set;}

    }

​

    class Quote : Document {

        double discount {get; set;}

    }

    public static void Main()

    {

        Document myDoc = new Document();

        myDoc.advance = 1000;

    }

}