#!/usr/bin/env python

from array import array
from math import sqrt
from sys import argv
from time import time
import sys
# import os
# import psutil
# process = psutil.Process(os.getpid())
xrange = range

def eval_A (i, j):
    return 1.0 / (((i + j) * (i + j + 1) >> 1) + i + 1)

def eval_A_times_u (u, resulted_list):
    u_len = len(u)
    local_eval_A = eval_A

    for i in xrange (u_len):
        partial_sum = 0

        j = 0
        while j < u_len:
            partial_sum += local_eval_A (i, j) * u[j]
            j += 1

        resulted_list[i] = partial_sum

def eval_At_times_u (u, resulted_list):
    u_len = len (u)
    local_eval_A = eval_A

    for i in xrange (u_len):
        partial_sum = 0

        j = 0
        while j < u_len:
            partial_sum += local_eval_A (j, i) * u[j]
            j += 1

        resulted_list[i] = partial_sum

def eval_AtA_times_u (u, out, tmp):
    eval_A_times_u (u, tmp)
    eval_At_times_u (tmp, out)

def main():
    # start = time()
    n = int(argv[1])
    u = array("d", [1]) * n
    v = array("d", [1]) * n
    tmp = array("d", [1]) * n
    local_eval_AtA_times_u = eval_AtA_times_u

    for dummy in xrange (10):
        local_eval_AtA_times_u(u, v, tmp)
        local_eval_AtA_times_u(v, u, tmp)

    vBv = vv = 0

    for ue, ve in zip(u, v):
        vBv += ue * ve
        vv  += ve * ve
    # end = time()
    # print (end - start)

    print("%0.9f" % (sqrt(vBv/vv)))


main()
