using System;

public class Program

{

    public static void Main(string[] args) {

        Console.Write("Enter a shape (circle, rectangle, triangle): ");

        string shape = Console.ReadLine().ToLower();

​

        double area = 0;

​

        switch (shape) {

            case "circle":

                Console.Write("Enter the radius: ");

                double radius = Convert.ToDouble(Console.ReadLine());

                area = Math.PI * Math.Pow(radius, 2);

                break;

            case "rectangle":

                Console.Write("Enter the length: ");

                double length = Convert.ToDouble(Console.ReadLine());

                Console.Write("Enter the width: ");

                double width = Convert.ToDouble(Console.ReadLine());

                area = length * width;

                break;

            case "triangle":

                Console.Write("Enter the base: ");

                double @base = Convert.ToDouble(Console.ReadLine());

                Console.Write("Enter the height: ");

                double height = Convert.ToDouble(Console.ReadLine());

                area = 0.5 * @base * height;

                break;

            default:

                Console.WriteLine("Invalid shape");

                break;

        }

​

        Console.WriteLine("The area of the {0} is {1}", shape, area);

    }

}

​