#include <bits/stdc++.h>
using namespace std;
#define ld long double
#define int long long
using vec_int = vector<int>;
using P = pair<int,int>;
using T = tuple<int,int,int>;
using ll = long long;
#define rep(i, n) for(int i = 0; i < (int)(n); i++)


int charToInt(char c){
    char zero_num = '0';
    return (int)c - (int)zero_num;
}
void cout_line(vector<int> &a){
    for(int i=0;i<a.size();i++){
        if(i<a.size()-1){
            cout<<a.at(i)<<" ";
        }else{
            cout<<a.at(i)<<endl;
        }
    }
}

signed main(){
    ld L; cin>>L;
    cout<<fixed<<setprecision(10)<<(L/3.)*(L/3.)*sqrt(3)/2./2.;


    return 0;
}