#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int n;
  double sum = 0;
  vector<int> list;
  cin >> n;
  for (int i = 0; i < n; i++) {
    int a;
    cin >> a;
    list.push_back(a);
  }
  for (int i = 0; i < n; i++) {
    if (i == 0)
      sum += (list[i] + list[i + 1]) / 2.0;
    else if (i >= 2)
      sum = (sum + list[i]) / 2.0;
  }
  cout << sum << endl;
  return 0;
}
