#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
  int n;
  long long cnt[3];
  cnt[0] = cnt[1] = 0;
  cin >> n;
  for (int i = 1, x; i <= n; ++i) {
    cin >> x;
    cnt[i % 2] += x / 2;
    cnt[(i + 1) % 2] += x - x / 2;
  }
  cout << min(cnt[0], cnt[1]) << endl;
  return 0;
}
