#include <bits/stdc++.h>

using namespace std;

void fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
}

int main() {
    fast_io();
    int n;
    cin >> n;
    long double ans = (15 + 5 * sqrtl(5)) / 12 * n * n * n;
    cout << setprecision(16) << fixed << ans << endl;
}