#include <cstdio>
#include <cmath>
#include <cstring>
#include <ctime>
#include <iostream>
#include <algorithm>
#include <set>
#include <vector>
#include <sstream>
#include <typeinfo>
#include <fstream>
#include <climits>
#include <functional>

#define REP(i,n) for(int i = 0;i < n;i++)

using namespace std;
typedef long long ll;

int main(void)
{
  cin.tie(0);
  ios::sync_with_stdio(false);

  double x;
  cin >> x;

  double total = M_PI * M_PI / 6;
  for(double i = 1;i < 1e7;i++){
    total -= 1.0 / (i * i);
  }

  for(double i = x + 1;i < 1e7;i += 1) {
    total += 1.0 / (i * i);
  }

  // cout << fixed << setprecision(10) << total << "\n";
  printf("%.14f\n", total);
  return 0;
}