#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  int arr[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  int count = 0;
  for (int i = 1; i < n - 1; i++) {
    if ((arr[i - 1] == 1 && arr[i + 1] == 1) && arr[i] == 0) {
      count++;
      arr[i + 1] = 0;
    }
  }
  cout << count << endl;
  ;
  return 0;
}
