import math

k_int = int(input())
pi_int = math.pi
count = 1
sum =0

if k_int > 0: 
	while(count <= k_int):
		sum = sum + (1/count)
		count=count+1
	sum = sum/k_int
else:
	sum = sum + (pi_int*pi_int/6)


print(sum)