#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<long long> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  long long black = 0;
  long long all = 0;
  for (int i = 0; i < n; i++) {
    long long now = a[i];
    all += now;
    if (i % 2 == 0) {
      black += (now - 1) / 2 + 1;
    } else {
      black += now / 2;
    }
  }
  long long white = all - black;
  cout << min(white, black) << endl;
}
