#include <bits/stdc++.h>
using namespace std;
const int maxn = 310000;
int a[maxn];
int ct[maxn], res[maxn];
int main() {
  int n;
  long long w = 0, b = 0;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    if (i % 2 == 1) {
      b += a[i] / 2;
      w += a[i] - (a[i] / 2);
    } else {
      w += a[i] / 2;
      b += a[i] - (a[i] / 2);
    }
  }
  cout << min(w, b) << endl;
  return 0;
}
