using System;

​

​

public abstract class Shape

{

    public abstract double CalculateArea();

}

​

public class Rectangle : Shape

{

    private double length;

    private double width;

    public Rectangle (double length, double width) // конструктор

    {

        this.length = length;

        this.width = width;

    }

    public override double CalculateArea()

    {

        return length * width;

    }

}

​

​

​

public class Program

{

    public static void Main()

    {

    }

}