#!/usr/bin/python3
# -*- coding: utf-8 -*-
# †
from collections import defaultdict
from itertools import combinations
from math import factorial

# i枚目のコストの期待値
def f(i):
    dic = defaultdict(int)
    dic[1] = factorial(i-1) * 2
    for p, q in combinations(range(1, i), r=2):
        dic[p*q] += factorial(i-2) * 2
    vals = sum(dic.values())
    res = sum(k*v for k, v in dic.items()) / vals
    return res

g = lambda n: sum(f(i) for i in range(1, n+1))

n = int(input())
res = g(n)
print('{:.10f}'.format(res))