using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp5
{
    class Program
    {

        static void Main(string[] args)
        {
            int R = int.Parse(Console.ReadLine());

            Console.WriteLine(R * 2 * 3.1415);


            }


        }
    }
}