#include <bits/stdc++.h>
using namespace std;
using i64 = long long int;
using ii = pair<int, int>;
using ii64 = pair<i64, i64>;
int main() {
  int n;
  scanf("%d", &n);
  i64 white = 0, black = 0;
  for (int i = 0; i < n; i++) {
    int x;
    scanf("%d", &x);
    white += x / 2;
    black += x / 2;
    if (i % 2 == 0)
      white += x % 2;
    else
      black += x % 2;
  }
  printf("%lld\n", min(white, black));
  return 0;
}
