import numpy as np
import math
import scipy as sp
import queue
import collections

MOD = 10 ** 9 + 7
INF = 10 ** 9

def main(kwargs):
    r = kwargs["r"]
    res = 2 * r * math.pi
    return res

if __name__ == "__main__":
    kwargs = {}

    cin = input().split()
    #kwargs["n"] = cin[0]
    #kwargs["n"], kwargs["k"] = cin
    kwargs["r"] = [int(i) for i in cin][0]
    #kwargs["n"], kwargs["k"] = [int(i) for i in cin]

    cout = main(kwargs)
    print(cout)