//
#include "bits/stdc++.h"
using namespace std;
#define pr1(a) for(auto b : a) cout << b << " ";cout << endl;
#define pr2(a) for(auto b : a){for(auto c:b) cout << c << " ";cout << endl;}
#define prc(...) for(auto b:{ __VA_ARGS__ }) cout << b << " ";cout << endl;
const long long mod=1000000007;

void solve()
{
    double n,ans;
    cin >> n;
    n/=3;
    ans=sqrt(3)*n*n/4;
    cout << fixed;
    cout.precision(10);
    cout<<ans<<endl;
    
}

int main()
{
    ios_base::sync_with_stdio(0);    cin.tie(0);    cout.tie(0);

    int tn = 1;
    //cin >> tn;
    while (tn--)
        solve();
}