#include <bits/stdc++.h>
const int maxn = 5e5 + 5;
using namespace std;
int a[maxn];
int main() {
  int n;
  scanf("%d", &n);
  long long ans = 0, sum = 0;
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &a[i]);
    int del = min((long long)a[i] >> 1, sum);
    ans += del;
    sum -= del;
    a[i] -= del << 1;
    ans += a[i] / 3;
    a[i] %= 3;
    sum += a[i];
  }
  cout << ans << endl;
  return 0;
}
