#include <bits/stdc++.h>
long long int a[300000];
int main() {
  long long int n, i, hei, bai;
  while (scanf("%lld", &n) != EOF) {
    hei = bai = 0;
    for (i = 0; i < n; i++) {
      scanf("%lld", &a[i]);
      if (i % 2) {
        hei += (a[i] + 1) / 2;
        bai += a[i] / 2;
      } else {
        bai += (a[i] + 1) / 2;
        hei += a[i] / 2;
      }
    }
    printf("%lld\n", bai < hei ? bai : hei);
  }
  return 0;
}
