#include <bits/stdc++.h>
using namespace std;
void init() { return; }
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  init();
  int n;
  cin >> n;
  long long int a[n];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  long long int ans = 0ll, prev = 0ll;
  for (int i = 0; i < n; i++) {
    long long int temp1 = min(a[i] / 2, prev);
    long long int temp2 = (a[i] - 2 * temp1) / 3;
    ans += temp1 + temp2;
    prev += a[i] - 3 * (temp1 + temp2);
  }
  cout << ans << endl;
  return 0;
}
