#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001

int main(){
	
	double L;
	cin>>L;
	
	L /= 3.0;
	
	double s = L*3.0 / 2.0;
	
	double ans = s;
	ans *= s-L;
	ans *= s-L;
	ans *= s-L;
	
	ans = sqrt(ans);
	
	cout<<fixed<<setprecision(15)<<ans<<endl;
	
	
	return 0;
}