using System;
using System.Collections.Generic;

class MainClass
{
   
    public static void Main(string[] args)
    {
        Console.WriteLine( (float)LnInt() * 2 * Math.PI);

    }

   
    public static int[] LnInts()
    {
        var vs = Console.ReadLine().Trim().Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
        var ins = new int[vs.Length];
        for (int i = 0; i < vs.Length; i++) ins[i] = int.Parse(vs[i]);
        return ins;
    }

    public static int LnInt()
    {
        var vs = Console.ReadLine().Trim();
        return int.Parse(vs);
    }

    public static string[] LnStrs()
    {
        var vs = Console.ReadLine().Trim().Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
        return vs;
    }
    public static string LnStr()
    {
        var vs = Console.ReadLine().Trim();
        return vs;
    }
}