#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n;
  cin >> n;
  long long a[n];
  for (long long i = 0; i < n; i++) cin >> a[i];
  long long b = 0, w = 0;
  int f = 0;
  for (long long i = 0; i < n; i++) {
    if (f == 0) {
      b += ((a[i] + 1) / 2);
      w += (a[i] / 2);
      f = 1;
    } else {
      w += ((a[i] + 1) / 2);
      b += (a[i] / 2);
      f = 0;
    }
  }
  cout << min(b, w) << '\n';
}
