# lniter
# Fig. 52.5
# inputs: 2 1e-10

import math

EPS = 1e-10

def logApprox(x):
  s = (x-1/x)/2
  c = (x+1/x)/2
  while abs(c-1) > EPS:
    c = math.sqrt((1+c)/2)
    s = s/c
  return s

x = float(input("x? "))
print(f"{logApprox(x):.11f}")