# expectHHHH.py 
# Fig. 38.5 
# input: 1000

n = int(input("n=? "))
probs = [0.0]*n
probs[0] = 1.0
probs[4] = 1/16
for i in range(5,n):
  probs[i] = probs[i-1] - probs[i-5]/32
# get expected no. of flips to reach n
en = 0
for i in range(4,n):
  en += i*probs[i]
print(f"Expected no. flips = {en:.6f}")