using System;
using System.Text;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
using static System.Math;
using System.Text.RegularExpressions;

namespace Yuzuki
{
    class Yukari
    {
        public static void Main(string[] args)
        {
            double r = double.Parse(Console.ReadLine());
            Console.WriteLine(r * PI);
        }
    }
}