#include <bits/stdc++.h>
using namespace std;
const long long N = 300010;
long long n, a[N];
int main() {
  long long s1 = 0, s2 = 0, i;
  scanf("%lld", &n);
  for (i = 1; i <= n; i++) {
    scanf("%lld", &a[i]);
    s1 += (i % 2 ? a[i] / 2 : (a[i] + 1) / 2);
    s2 += (i % 2 ? (a[i] + 1) / 2 : a[i] / 2);
  }
  cout << min(s1, s2);
  return ~~(0 - 0);
}
