#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, w, b, a;
  w = b = 0;
  cin >> n;
  while (n--) {
    cin >> a;
    w += (a / 2);
    b += (a / 2);
    if (a % 2) {
      if (n % 2)
        w++;
      else
        b++;
    }
  }
  cout << min(w, b) << endl;
  return 0;
}
