#include <bits/stdc++.h>
using namespace std;
long long n, x, a1, a2;
int main() {
  scanf("%lld", &n);
  for (int i = 1; i <= n; ++i) {
    scanf("%lld", &x);
    if (i % 2 == 0) {
      a1 += (x + 1) / 2;
      a2 += x / 2;
    } else {
      a1 += x / 2;
      a2 += (x + 1) / 2;
    }
  }
  printf("%lld\n", min(a1, a2));
  return 0;
}
