#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
#define rep(i,n) for(int i=0; i<n; ++i)

int main(){
    double L;
    cin >> L;

    double S, l;
    l = L / 3.0;
    S = l * (sqrt(3) * l * 0.5) * 0.5;

    printf("%.9f\n", S); 

    return 0;
}