#include <bits/stdc++.h>
using namespace std;
int a[100001];
int ans;
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  for (int i = 1; i < n; i++) {
    a[i] = (a[i] + a[i - 1]);
    ans += a[i] / 2;
  }
  cout << ans / 2;
}
