#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>

#define mkp make_pair
#define mkt make_tuple
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define all(v) v.begin(), v.end()
using namespace std;
typedef long long ll;
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
template <class T>
void chmin(T &a, const T &b) {
    if (a > b) a = b;
}
template <class T>
void chmax(T &a, const T &b) {
    if (a < b) a = b;
}

void solve() {
    double L;
    cin >> L;

    double base_len = L / 3;
    double area = base_len * sqrt(pow(base_len, 2) - pow(base_len / 2, 2)) / 2;
    cout << fixed << setprecision(15) << area << endl;
}

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

    solve();

    return 0;
}