#include <bits/stdc++.h>
using namespace std;
int a[300010];
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &a[i]);
  }
  long long ans = 0;
  vector<int> v;
  for (int i = 1; i <= n; ++i) {
    ans += a[i] / 2;
    if (a[i] % 2 != 0) {
      v.push_back(i);
    }
  }
  stack<int> q;
  if (v.size() == 0) {
    cout << ans << endl;
    return 0;
  }
  q.push(v[0]);
  for (int i = 1; i < v.size(); ++i) {
    if (q.size() && (v[i] - q.top()) % 2 != 0) {
      q.pop();
      ans++;
      continue;
    }
    q.push(v[i]);
  }
  cout << ans << endl;
  return 0;
}
