#include <bits/stdc++.h>
using namespace std;
const int oo = 0x3f3f3f3f;
const long long ooo = 9223372036854775807ll;
const int _cnt = 1000 * 1000 + 7;
const int _p = 1000 * 1000 * 1000 + 7;
const int N = 300005;
const double PI = acos(-1.0);
const double eps = 1e-9;
int o(int x) { return x % _p; }
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
int lcm(int a, int b) { return a / gcd(a, b) * b; }
void file_put() {
  freopen("filename.in", "r", stdin);
  freopen("filename.out", "w", stdout);
}
int n, a[N], L, t;
long long ret = 0;
int main() {
  scanf("%d", &n);
  for (int i = (1); i <= (n); ++i) scanf("%d", &a[i]), ret += a[i];
  for (int i = (1); i <= (n); ++i) {
    if (a[i] >= 2)
      t = ((a[i] >> 1) < (L) ? (a[i] >> 1) : (L)), a[i] -= t << 1, L -= t;
    a[i] %= 3, L += a[i];
  }
  printf("%lld\n", (ret - L) / 3);
  return 0;
}
