import sys
from itertools import permutations
from heapq import heappop,heappush
from collections import deque
import random
import bisect

input = lambda :sys.stdin.readline().rstrip()
mi = lambda :map(int,input().split())
li = lambda :list(mi())

from math import log
from math import log1p

def isqrt(n):
    t = int(n**.5)
    for i in range(t-2,t+3)[::-1]:
        if i*i <= n:
            return i

"""
d(n) = prod (1+p+p^2+...+p^e)
d(n)/n = prod (1+p^-1+p^-2+...+p^-e)

ij <= N に対する 1/ijの総和
"""

M = 10**6
memo_cumulateive_inv_sum = [0] * (M+1)
for n in range(1,M+1):
    memo_cumulateive_inv_sum[n] = memo_cumulateive_inv_sum[n-1] + 1/n

def approximate(N):
    return log(N+0.5) + 0.57721566490152286060

def calc_cumulative_inv_sum(N):
    M = 10**6
    if N <= M:
        #print(N,memo_cumulateive_inv_sum[N]-approximate(N))
        return memo_cumulateive_inv_sum[N]
    return log(N+1/2) + 0.57721566490152286060

def calc_cumulative_inv_range_sum(l,r):
    return log1p((2*r-2*l+2)/(2*l-1))
    return log((2*r+1)/(2*l-1))

#M = 1000
def solve(N):
    B = isqrt(N)
    res = 0
    for n in range(1,B+1):
        res += 2*calc_cumulative_inv_sum(N//n)/n
    
    #print(res,calc_cumulative_inv_sum(B)**2)
    res -= calc_cumulative_inv_sum(B)**2
    
    
        
    return res

print(solve(int(input())))