#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, a[109];
  while (cin >> n) {
    for (int i = 1; i <= n; i++) {
      cin >> a[i];
    }
    int count = 0;
    for (int i = 2; i < n; i++) {
      if (a[i] == 0) {
        if (a[i - 1] == 1 && a[i + 1] == 1) {
          a[i + 1] = 0;
          count++;
        }
      }
    }
    cout << count << endl;
  }
}
