#include <bits/stdc++.h>
using namespace std;
const int MAXN = 3e5 + 10;
int a[MAXN];
int main() {
  int n;
  scanf("%d", &n);
  long long b = 0, w = 0;
  for (int i = 1; i <= n; i++) {
    scanf("%d", a + i);
    if (i & 1) {
      b += a[i] / 2;
      w += (a[i] + 1) / 2;
    } else {
      b += (a[i] + 1) / 2;
      w += a[i] / 2;
    }
  }
  printf("%lld\n", min(b, w));
}
