#include <bits/stdc++.h>
using namespace std;
long long n, w, b;
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cout.precision(40);
  cin >> n;
  for (long long i = 0, a; i < n; ++i) {
    cin >> a;
    if (i & 1)
      w += a / 2, b += (a + 1) / 2;
    else
      b += a / 2, w += (a + 1) / 2;
  }
  cout << min(w, b);
}
