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

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

int main() {
	int L;
	cin >> L;
	long double a = L;
	a /= 3;
	long double ans = a * a * sqrt(3) / 4;
	cout << fixed << setprecision(12) << ans << endl;
}