#include <bits/stdc++.h>
using namespace std;
using lint = long long;
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V< V<T> >;

int main() {
  cin.tie(nullptr); ios::sync_with_stdio(false);
  constexpr double pi = acos(-1);
  double x; cin >> x;
  double res = pi * pi / 6;
  for (double n = 1; n <= 1e8; ++n) {
    res -= x * (x + 2 * n) / pow(n * (x + n), 2);
  }
  cout << fixed << setprecision(15) << res << '\n';
}