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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            double e;
            e = double.Parse(Console.ReadLine());
            double p = 0;
            double s = 0;
            int br = 1;
            double c = 1;
            while (Math.Abs(c) <= e)
            {
                c = (double)1 / br;
                br += 2;
                br *= -1;
                s += c;
            }
            p = s * 4;
            Console.WriteLine(p.ToString("0.00"));
            Console.ReadLine();
        }
    }
}