#include <bits/stdc++.h>
using namespace std;
const long long int infinity = 9e18;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int n;
  cin >> n;
  long long int total = 0, odd = 0, even = 0;
  long long int k;
  for (int i = 0; i < n; i++) {
    cin >> k;
    total += k / 2;
    if (k & 1) {
      if (i & 1)
        odd++;
      else
        even++;
    }
  }
  cout << (total + min(odd, even));
  return 0;
}
