public static void Main()
const double Pi = 3.141592653589793238462;
Console.WriteLine(" Enter the value of Radius: ");
r = Convert.ToDouble(Console.ReadLine());
Console.WriteLine(4 * Pi * r);
Console.WriteLine(4 * Pi * r);
Console.WriteLine(" Do you want to try again [Y/N] ");
opt = Convert.ToChar(Console.ReadLine());
if (opt =='Y' || opt == 'y')