#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  double a, b;
  cin >> n;
  vector<int> vec(n);
  for (int i = 0; i < n; i++) {
    cin >> vec.at(i);
  }
  sort(vec.begin(), vec.end());
  if (n % 2 == 0) {
    for (int i = 0; i < n - 1; i + 2) {
      a = (vec.at(i) + vec.at(i + 1)) / 2;
      b += a;
    }
  }
  if (n % 2 == 1) {
    for (int i = 0; i < n - 2; i + 2) {
      a = (vec.at(i) + vec.at(i + 1)) / 2;
      b += a;
    }
    b += vec.at(n);
  }
  cout << b << endl;
}
