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