#include <bits/stdc++.h>
using namespace std;
signed main() {
  long long n, m;
  cin >> n;
  long long res1 = 0;
  long long res2 = 0;
  for (long long i = 1; i <= n; i++) {
    long long x;
    cin >> x;
    if (i % 2) {
      res1 += (x + 1) / 2;
      res2 += x / 2;
    } else {
      res2 += (x + 1) / 2;
      res1 += x / 2;
    }
  }
  cout << min(res1, res2) << endl;
  return 0;
}
