#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n;
  cin >> n;
  long long* a = new long long[n];
  long long b = 0, w = 0;
  for (long long i = 0; i < n; i++) {
    cin >> a[i];
    b += (a[i]) / 2;
    w += a[i] / 2;
    if (a[i] % 2 == 1) {
      if (i % 2 == 0)
        b++;
      else
        w++;
    }
  }
  cout << min(b, w) << endl;
}
