#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
template<class T>bool chmax(T& a, const T& b) { if (a < b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T& a, const T& b) { if (b < a) { a = b; return 1; } return 0; }
typedef long long ll;
using ll = long long;
using P = pair<int,int>;
 
int main() {
  double l;
  cin >> l;
  double a = l * l;
  double b =  12 * sqrt(3);
  double ans = a / b;
  cout << ans << endl;
}