#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[105];
  int cnt = 0;
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 2; i <= n - 1; i++) {
    if (a[i] == 0 && a[i - 1] == a[i + 1] && a[i + 1] == 1) {
      a[i + 1] = 0;
      cnt++;
    }
  }
  cout << cnt << endl;
  return 0;
}
