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