#include <bits/stdc++.h>
using namespace std;
long long n, a[300005], tp, lt, ans;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) scanf("%lld", &a[i]);
  for (int i = 1; i <= n; i++) {
    tp = min(a[i] / 2, lt);
    lt -= tp;
    a[i] -= tp * 2;
    ans += tp + a[i] / 3;
    lt += a[i] % 3;
  }
  cout << ans;
}
