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

namespace PaizaTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var r = double.Parse(Console.ReadLine());

            Console.WriteLine(2.0 * r * Math.PI);
#if DEBUG
            Console.ReadLine();
#endif
        }
    }
}
