#include <bits/stdc++.h>
using namespace std;
const int maxn = 5e5 + 10;
long long cnt[15];
int main() {
  int n;
  scanf("%d", &n);
  int pre = -1;
  long long ans = 0;
  for (int i = 1; i <= n; i++) {
    long long temp;
    scanf("%lld", &temp);
    cnt[i % 2] += temp / 2;
    cnt[(i + 1) % 2] += temp - temp / 2;
  }
  printf("%lld\n", min(cnt[0], cnt[1]));
}
