#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e6 + 10;
int n, v[maxn];
long long b, w;
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d", &v[i]);
  for (int i = 1; i <= n; i++)
    if (i & 1) {
      w += (v[i] + 1) / 2;
      b += v[i] / 2;
    } else {
      b += (v[i] + 1) / 2;
      w += v[i] / 2;
    }
  printf("%lld\n", min(b, w));
  return 0;
}
