#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
int n, a[maxn];
int main() {
  long long x;
  cin >> n;
  bool f = 0;
  long long ans = 0, odd = 0, even = 0;
  for (int i = 1; i <= n; i++) {
    cin >> x;
    ans += x / 2;
    if (x & 1) {
      if (i & 1)
        odd++;
      else
        even++;
    }
  }
  cout << ans + min(odd, even) << endl;
}
