#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;
using ll = long long;

#include<math.h>
#include<iomanip>
int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int l;
    cin>>l;
    long double use = l;
    use /= 3;
    long double ans = use * use * sqrt(3) / 4;
    cout<<setprecision(30)<<fixed<<ans<<endl;
}