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