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

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            var R = Console.ReadLine().Split().Select(s=>int.Parse(s)).ToArray()[0];
            Console.WriteLine(Math.PI * 2 * R);
        }
    }
}