using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("");Console.Write("Enter circle radius. r = "); 

​

var r = double.Parse(Console.ReadLine()); 

​

Console.WriteLine("Area = " + Math.PI * r * r);  

​

  // Math.PI - вградена в C# константа за π 

​

Console.WriteLine("Perimeter = " + 2 * Math.PI * r); 

    }

}