#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int ans = 0;
  for (int i = 1; i < n - 1; i++) {
    if (a[i] == 0 && a[i - 1] == 1 && a[i + 1] == 1) {
      a[i + 1] = 0;
      ans++;
    }
  }
  cout << ans << endl;
}
