#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k = 0;
  cin >> n;
  int arr[n + 2];
  arr[0] = arr[n + 1] = 0;
  for (int i = 1; i <= n; i++) cin >> arr[i];
  for (int i = 1; i <= n; i++)
    if (arr[i - 1] == 1 && arr[i + 1] == 1 && arr[i] == 0) arr[i + 1] = 0, k++;
  cout << k;
  return 0;
}
