#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 count = 0;
  for (int i = 1; i < n - 1; i++) {
    if (arr[i - 1] == 1 && arr[i] == 0 && arr[i + 1] == 1) {
      arr[i + 1] = 0;
      count++;
    }
  }
  cout << count;
  return 0;
}
