#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  long long w = 0, b = 0;
  for (int i = 0; i < n; ++i) {
    long long x;
    scanf("%lld", &x);
    if (i & 1) {
      w += (x + 1) / 2;
      b += (x / 2);
    } else {
      w += x / 2;
      b += (x + 1) / 2;
    }
  }
  cout << min(w, b) << endl;
  return 0;
}
