//
// ABC163A
// URL : https://atcoder.jp/contests/abc162/tasks/abc163_a
// 
using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;

namespace AtCoder
{
    class ABC163A
    {
        static void Main(string[] args)
        {
            // 整数の入力
            long r = long.Parse(Console.ReadLine());

            long ans = r * 2.0 * Math.PI;

            Console.WriteLine(ans.ToString());
            Console.Out.Flush();
        }
    }
}

