#include <bits/stdc++.h>
using namespace std;
const long long MaxN = 3e5 + 10;
long long a[MaxN];
int main() {
  long long n;
  cin >> n;
  for (long long i = 0; i < n; i++) cin >> a[i];
  long long sum1 = 0, sum2 = 0;
  for (long long i = 0; i < n; i++) {
    if (i % 2 == 0) {
      sum1 += a[i] / 2;
      sum2 += (a[i] + 1) / 2;
    } else {
      sum1 += (a[i] + 1) / 2;
      sum2 += a[i] / 2;
    }
  }
  cout << min(sum1, sum2);
}
