#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  long long temp;
  long long white = 0, black = 0;
  for (int i = 0; i < n; i++) {
    cin >> temp;
    white += temp / 2;
    black += (temp + 1) / 2;
    swap(white, black);
  }
  cout << min(white, black) << endl;
  return 0;
}
