#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> a;
  for (int i = 0; i < n; i++) {
    int temp;
    cin >> temp;
    a.push_back(temp);
  }
  int k = 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;
      k++;
    }
  }
  cout << k;
}
