#include <bits/stdc++.h>

#define all(x) begin(x), end(x)

using namespace std;
using i32 = int;
using i64 = long long;
using ld = long double;

template <typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {return a < b and (a = b, true);}

template <typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {return a > b and (a = b, true);}

const i64 supl = LONG_LONG_MAX / 2 - 100;
const i32 supi = INT_MAX / 2 - 100;

void main_() {
    i32 n; cin >> n;
    ld ans = (ld)(15 + 5 * sqrt(5)) / 12.0 * (ld)n * n * n;
    cout << fixed << setprecision(12);
    cout << ans << endl;
}

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

    main_();

    return 0;
}