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