#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  long long sum = 0, sum2 = 0;
  while (cin >> n) {
    sum = 0, sum2 = 0;
    for (int i = 1; i <= n; i++) {
      int k;
      cin >> k;
      sum += k / 2;
      sum2 += k / 2;
      if (i & 1) {
        if (k & 1) sum++;
      } else if (k & 1)
        sum2++;
    }
    cout << min(sum, sum2) << endl;
  }
}
