#!/usr/bin/env python
# -*- coding: utf-8 -*-
#
# FileName: 	A
# CreatedDate:  2020-04-19 21:06:34 +0900
# LastModified: 2020-04-19 21:08:30 +0900
#


import os
import sys
import numpy as np
import pandas as pd


def main():
    r = int(input())
    print(2*r*3.1415926)


if __name__ == "__main__":
    main()
