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