#!/usr/bin/python
# -*- coding: UTF-8 -*-

"""
题目：编写一个函数，输入n为偶数时，调用函数求
1/2+1/4+...+1/n,
当输入n为奇数时，调用函数1/1+1/3+...+1/n(利用指针函数)
"""
def peven(n):
	i = 0
	s = 0.0
	for i in range(2,n + 1,2):
		s +=  1.0 / i
	return s

def podd(n):
	s = 0.0
	for i in range(1,n + 1,2):
		s += 1 / i
	return s

def dcall(fp,n):
	s = fp(n)
	return s

if __name__ == '__main__':
	n = int(raw_input('input a number:\n'))
	if n % 2 == 0:
		sum = dcall(peven,n)
	else:
		sum = dcall(podd,n)
	print sum

	pass