using System;

public class Program

{

    static double Obim(double r)

    {

        double o = 2*r*Math.PI;

        return o;

    }

     static void Main()

    {

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

         double ob = Obim(r);

         Console.WriteLine(ob);

         Console.ReadLine();

    }

}