#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,m,n) for(int i=m; i<n; ++i)
#define repl(i,m,n) for(ll i=m; i<n; ++i)

double area(double r){
    double h = sqrt(3.0) * r / 2.0;
    return r * h / 2.0;
}

int main(){
    double L;
    cin >> L;
    double ans = area(L/3.0);
    printf("%.22f\n", ans);
    return 0;
}