#include <bits/stdc++.h>
using namespace std;
const int INF = 1 << 30;
const long long int MAX = 1e9 + 7;
void array_show(int *array, int array_n, char middle = ' ') {
  for (int i = 0; i < array_n; i++)
    printf("%d%c", array[i], (i != array_n - 1 ? middle : '\n'));
}
void array_show(long long int *array, int array_n, char middle = ' ') {
  for (int i = 0; i < array_n; i++)
    printf("%lld%c", array[i], (i != array_n - 1 ? middle : '\n'));
}
void array_show(vector<int> &vec_s, int vec_n = -1, char middle = ' ') {
  if (vec_n == -1) vec_n = vec_s.size();
  for (int i = 0; i < vec_n; i++)
    printf("%d%c", vec_s[i], (i != vec_n - 1 ? middle : '\n'));
}
void array_show(vector<long long int> &vec_s, int vec_n = -1,
                char middle = ' ') {
  if (vec_n == -1) vec_n = vec_s.size();
  for (int i = 0; i < vec_n; i++)
    printf("%lld%c", vec_s[i], (i != vec_n - 1 ? middle : '\n'));
}
vector<pair<int, int> > v1;
int main() {
  int n, m;
  int i, j, k;
  long long int a, b, c;
  cin >> n;
  b = 0;
  for (i = 0; i < n; i++) {
    cin >> a;
    b += a;
    if (v1.empty())
      v1.push_back(make_pair(a, 1));
    else {
      pair<int, int> &pa = v1.back();
      if (pa.first == a)
        pa.second++;
      else
        v1.push_back(make_pair(a, 1));
    }
  }
  reverse(v1.begin(), v1.end());
  a = 0;
  for (i = 0; i < v1.size(); i++) {
    if (v1[i].second % 2 == 0) continue;
    if ((v1[i].first - a) % 2 == 0) {
      if (a > 0) a--;
    } else
      a++;
  }
  b -= a / 2;
  cout << b / 2 << endl;
}
