import numpy as np
#import math
#from fractions import gcd

def main():

    ##get int
    first_input = list(map(lambda x:int(x), input().split(" ")))
    #second_input = list(map(lambda x:int(x), input().split(" ")))

    ##get float
    #first_input = list(map(lambda x:float(x), input().split(" ")))
    #second_input = list(map(lambda x:float(x), input().split(" ")))

    ##get string
    #first_input = input().split(" ")

    R = first_input[0]

    print(R*2*np.pi)

if __name__ == '__main__':
    main()
