#include <bits/stdc++.h>
using namespace std;
long long a[300009], b[2], i, n, flag, sum, c;
int main() {
  cin >> n;
  for (i = 0; i < n; i++) {
    cin >> a[i];
    b[i % 2] = b[i % 2] + a[i] / 2 + a[i] % 2;
    b[(i + 1) % 2] = b[(i + 1) % 2] + a[i] / 2;
  }
  sum = min(b[0], b[1]);
  cout << sum << endl;
}
