#include <bits/stdc++.h>
using namespace std;
int n, f[1000005];
long long ans, las;
int gets() {
  int v = 0;
  char ch;
  bool f = 0;
  while (!isdigit(ch = getchar()))
    if (ch == '-') f = 1;
  v = ch - 48;
  while (isdigit(ch = getchar())) v = (v << 1) + (v << 3) + ch - 48;
  return f ? -v : v;
}
int main() {
  n = gets();
  for (int i = 1; i <= n; i++) {
    f[i] = gets();
    long long tmp = min(las, (long long)f[i] / 2);
    las -= tmp;
    f[i] -= tmp * 2;
    ans += tmp;
    tmp = f[i] / 3;
    ans += tmp;
    las += f[i] % 3;
  }
  cout << ans << endl;
}
