#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  long long int arr[n];
  for (int i = 0; i < n; i++) cin >> arr[i];
  long long int w = 0;
  long long int b = 0;
  for (int i = 0; i < n; i++) {
    if (i % 2 == 0) {
      w += arr[i] / 2;
      b += arr[i] - (arr[i] / 2);
    } else {
      b += arr[i] / 2;
      w += arr[i] - (arr[i] / 2);
    }
  }
  cout << min(b, w) << endl;
  return 0;
}
