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

#define ll long long
#define rep(i, n) for (int i = 0; i < (n); i++)
#define P pair<int, int>

int main() {
  double l;
  cin >> l;
  l /= 3;
  cout << fixed << setprecision(15) << sqrt(3)*l*l/4 << endl;


}