#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n;
  cin >> n;
  vector<long long> v(2);
  for (long long i = 0; i < n; ++i) {
    long long x;
    cin >> x;
    if (i & 1) {
      v[1] += (x + 1) / 2;
      v[0] += x / 2;
    } else {
      v[0] += (x + 1) / 2;
      v[1] += x / 2;
    }
  }
  cout << min(v[0], v[1]) << endl;
}
