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