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