#include <bits/stdc++.h>
using namespace std;

int main() {
  iostream::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  cout << fixed << setprecision(20);
  double n;
  cin >> n;
  double v = 5.0 * (3 + sqrt(5)) / 12.0 * n * n * n;
  cout << v << '\n';


  return 0;
}