#include <bits/stdc++.h>
#include<iostream>
#include<map>
#include<vector>
#include <algorithm>
#include<math.h>
#include <iomanip>
#include<set>
#include <numeric>
#include<string>
using ll = long long;
using namespace std;

int main()
{
    ll n;
    cin >> n;
    cout << fixed << setprecision(8);
    cout << 5*(3 + (double)pow(5, 0.5)) * n*n*n /12 << endl;
}