/* -*- coding: utf-8 -*-
 *
 * 454.cc: No.454 逆2乗和 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 2000000;

/* typedef */

typedef long double ld;

/* global variables */

/* subroutines */

/* main */

int main() {
  ld x;
  cin >> x;

  ld sum = 0.0;
  for (int n = 1; n <= MAX_N; n++) {
    ld nx = x + n;
    sum += (ld)1.0 / (nx * nx);
  }
  sum += (ld)1.0 / MAX_N;
  
  printf("%.12Lf\n", sum);
  return 0;
}