#include <bits/stdc++.h>
using namespace std;
vector<long long> a;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n, x, t = 0, m, ew = 0, eb = 0;
  cin >> n;
  a.resize(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    t += a[i];
    a[i] %= 2;
    if (i % 2) {
      ew += a[i];
    } else {
      eb += a[i];
    }
  }
  cout << (t - abs(ew - eb)) / 2;
  return 0;
}
