#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  double ans = 0;
  cin >> n;
  double v[n];
  for (int i = 0; i < n; i++) {
    cin >> v[i];
  }
  for (int i = 0; i < (n - 1); i++) {
    if (i == 0)
      ans = (v[i] + v[++i]) / 2;
    else
      ans = (ans + v[i]) / 2;
  }
  cout << ans << endl;
}
