#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<double> v;
  for (int i = 0; i < n; ++i) {
    int z;
    cin >> z;
    v.emplace_back(z);
  }
  double maxAll = 0.0;
  for (vector<double>::iterator i = v.begin(); i != v.end(); ++i) {
    for (vector<double>::iterator j = next(i); j != v.end(); ++j) {
      double value = (*i + *j) / 2.0;
      for (vector<double>::iterator k = v.begin(); k != v.end(); ++k) {
        if ((k == i) || (k == j)) {
          continue;
        }
        double value2 = (value + *k) / 2.0;
        if (value2 > maxAll) {
          maxAll = value2;
        }
      }
    }
  }
  cout << maxAll;
  return 0;
}
