#include <bits/stdc++.h>
using namespace std;
int INF = 1e9 + 7;
long long INFLL = 1e17;
const int maxn = 3e5 + 5;
int n;
vector<int> A;
int main() {
  cin >> n;
  A = vector<int>(n);
  long long odd = 0, even = 0;
  for (int i = 0; i < (n); ++i) {
    cin >> A[i];
    odd += A[i] / 2;
    even += A[i] / 2;
    if (A[i] % 2) {
      if (i % 2) {
        odd++;
      } else {
        even++;
      }
    }
  }
  printf("%lld\n", min(odd, even));
}
