#include <bits/stdc++.h>
using namespace std;
void vivod(string s, vector<long long> a) {}
signed main() {
  cin.tie(0), cout.tie(0), ios_base::sync_with_stdio(0);
  long long n;
  cin >> n;
  vector<long long> a(n, 0);
  for (long long i = 0; i < n; i++) cin >> a[i];
  vector<long long> cnt = {0, 0};
  for (long long i = 0; i < n; i++) {
    cnt[(i % 2)] += a[i] / 2;
    cnt[(i % 2) ^ 1] += a[i] / 2 + a[i] % 2;
  }
  vivod("lca", {1, 2, 3});
  cout << min(cnt[0], cnt[1]) << '\n';
}
