#include <cmath>
#include <iomanip>
#include <iostream>

int main() {
    int64_t N; std::cin >> N;
    long double vol_1 = (15 + 5 * sqrtl(5)) / 12;
    std::cout << std::fixed << std::setprecision(18) << N * N * N * vol_1 << std::endl;
}