# sumInvSquares.py 
# Fig. 6.8a  
# input: 500 or 1000 or 1500

n = int(input("n=? "))
sum = 0.0
for i in range(n, 0 ,-1):
  sum += 1.0/(i*i)
sum += 1.0/(n+0.5)
print(sum)