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