#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, x;
  long long b = 0, w = 0;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    scanf("%d", &x);
    b += (x / 2);
    w += (x / 2);
    if (i & 1)
      b += (x & 1);
    else
      w += (x & 1);
  }
  printf("%I64d\n", min(b, w));
}
