#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int i, j, k, n, m, t, sum = 0, ans = 0;
  bool f = 0, flag = 0;
  string s;
  cin >> n;
  long long int a[n];
  for (i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (i = 1; i < n - 1; i++) {
    if (a[i] == 0 && a[i - 1] == 1 && a[i + 1] == 1) {
      a[i + 1] = 0;
      sum++;
      i++;
    }
  }
  cout << sum << endl;
  return 0;
}
