using System;

​

public class Program

{

    public static void Main()

    {

        for (;;)

        {

            Console.WriteLine("Enter Radius: ");

            double r;

            r = Convert.ToDouble(Console.ReadLine());

            const double pi = 3.14159;

            double areaCircle = pi * r * r;

            if (r >= 0)

            {

                // constant declaration 

                Console.WriteLine("Radius: {0}, Area: {1}", r, areaCircle);

            }

​

            if (r < 0)

            {

                Console.WriteLine("Invalid Value! Please enter a positive integer!");

            }

        }

    }

}