#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  int a[101];
  int num = 0;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  for (int i = 1; i < n; ++i) {
    if (a[i] == 0 && a[i - 1] == 1 && a[i + 1] == 1) {
      num++;
      if (a[i + 2] == 0) {
        i += 2;
      }
    }
  }
  cout << num;
  return 0;
}
