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

int main(){
  double n;
  cin >> n;
  double ans = (15.0+5.0*sqrt(5))/12.0*n*n*n;
  cout << fixed << setprecision(12) << ans << endl;
  return 0;
}