#include <bits/stdc++.h>
using namespace std;
using lint = long long;
const lint mod = 256;
template <class T>
inline void YES(T condition) {
  if (condition)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
template <class T>
inline void Yes(T condition) {
  if (condition)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
template <class T>
inline void POSS(T condition) {
  if (condition)
    cout << "POSSIBLE" << endl;
  else
    cout << "IMPOSSIBLE" << endl;
}
template <class T>
inline void Poss(T condition) {
  if (condition)
    cout << "Possible" << endl;
  else
    cout << "Impossible" << endl;
}
template <class T>
inline void First(T condition) {
  if (condition)
    cout << "First" << endl;
  else
    cout << "Second" << endl;
}
template <class T = string, class U = char>
int character_count(T text, U character) {
  int ans = 0;
  for (U i : text) {
    ans += (i == character);
  }
  return ans;
}
lint power(lint base, lint exponent, lint module) {
  if (exponent % 2) {
    return power(base, exponent - 1, module) * base % module;
  } else if (exponent) {
    lint root_ans = power(base, exponent / 2, module);
    return root_ans * root_ans % module;
  } else {
    return 1;
  }
}
struct position {
  int y, x;
};
position mv[4] = {{0, -1}, {1, 0}, {0, 1}, {-1, 0}};
template <class T, class U>
string to_string(pair<T, U> x) {
  return to_string(x.first) + "," + to_string(x.second);
}
string to_string(string x) { return x; }
template <class itr>
void array_output(itr start, itr goal) {
  string ans;
  for (auto i = start; i != goal; i++) ans += to_string(*i) + " ";
  if (!ans.empty()) ans.pop_back();
  cout << ans << endl;
}
template <class itr>
void cins(itr first, itr last) {
  for (auto i = first; i != last; i++) {
    cin >> (*i);
  }
}
template <class T>
T gcd(T a, T b) {
  if (a && b) {
    return gcd(min(a, b), max(a, b) % min(a, b));
  } else {
    return a;
  }
}
template <class T>
T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
struct combination {
  vector<lint> fact, inv;
  combination(int sz) : fact(sz + 1), inv(sz + 1) {
    fact[0] = 1;
    for (int i = 1; i <= sz; i++) {
      fact[i] = fact[i - 1] * i % mod;
    }
    inv[sz] = power(fact[sz], mod - 2, mod);
    for (int i = sz - 1; i >= 0; i--) {
      inv[i] = inv[i + 1] * (i + 1) % mod;
    }
  }
  lint C(int p, int q) const {
    if (q < 0 || p < q) return 0;
    return (fact[p] * inv[q] % mod * inv[p - q] % mod);
  }
};
template <class itr>
bool next_sequence(itr first, itr last, int max_bound) {
  itr now = last;
  while (now != first) {
    now--;
    (*now)++;
    if ((*now) == max_bound) {
      (*now) = 0;
    } else {
      return true;
    }
  }
  return false;
}
int main() {
  int n;
  cin >> n;
  lint a[n];
  cins(a, a + n);
  lint sum[n];
  sum[0] = a[0];
  for (int i = 1; i < n; i++) {
    sum[i] = sum[i - 1] + a[i];
  }
  lint ans = 0, remaining = 0;
  for (int i = n - 1; i >= 0; i--) {
    lint limit = (sum[i] - remaining) / 3;
    if (limit >= a[i] / 2) {
      remaining += a[i] / 2;
      ans += a[i] / 2;
      if (remaining > 0) {
        remaining -= a[i] % 2;
      }
    } else {
      ans += limit;
      break;
    }
  }
  cout << ans << endl;
}
