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