#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  long long sum = 0, a = 0, o = 0;
  for (int i = 0; i < n; i++) {
    cin >> a;
    sum += a;
    if (a & 1)
      o += ((a + (i & 1)) / 2);
    else
      o += a / 2;
  }
  cout << min(o, sum - o);
}
