#!/usr/bin/python2
# -*- coding: utf-8 -*-
# †
from math import pi, sin

def trigamma(x):
    small = 1e-4
    large = 8
    f1 = float(1)

    if x <= 0 and int(x) == x:
        return float('inf')
    if x < 0:
        val = pi / sin(-pi * x)
        return -trigamma(1-x) + val*val
    if x <= small:
        c = pi * pi / 6 # Zeta(2)
        c1 = -2.404113806319188570799476  # -2 Zeta(3)
        return f1 / (x*x) + c + c1*x
    val = 0.
    while x < large:
        val += f1 / (x*x)
        x += 1
    if x >= large:
        r = f1 / (x*x)
        bs = [f1, f1/6, -f1/30, f1/42, -f1/30, f1*5/66]
        val += .5*r + reduce(lambda by, bx: bx + r*by, reversed(bs)) / x
    return val

x = float(raw_input())
res = trigamma(x+1)
print '{:.15f}'.format(res)