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

namespace Main
{
    class Program
    {
        static void Main(string[] args)
        {
            //ここから
            //string[] str = Console.ReadLine().Split(' ');
            //int n = int.Parse(Console.ReadLine());
            int n = int.Parse(Console.ReadLine());

            Console.WriteLine((long)n * 3.1415926535);


            //ここまで
        }

        static long Max(long a, long b)
        {
            if ( a >= b) return a;
            else return b;
        }
    }

}
