#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef unsigned long long ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}

constexpr ll mod=1e9+7;

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    double d; cin >> d;
    d/=3.0;
    printf("%.9f\n",sqrt(3)/4.0*d*d);
}