#include <bits/stdc++.h>
using namespace std;
long long min(long long a, long long b) { return a < b ? a : b; }
long long max(long long a, long long b) { return a > b ? a : b; }
signed main() {
  long long n, w = 0, b = 0, x, c = 0;
  cin >> n;
  for (long long i = 0; i < n; i++) {
    cin >> x;
    if (c) {
      w += x / 2;
      b += x - x / 2;
    } else {
      b += x / 2;
      w += x - x / 2;
    }
    c = c ^ 1;
  }
  cout << min(w, b) << endl;
}
