#include <bits/stdc++.h>
using namespace std;
int a[101];
int res = 0;
int n;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 2; i <= n - 1; i++) {
    if (a[i - 1] && a[i + 1] && !a[i]) {
      ++res;
      a[i - 1] = 0;
      a[i + 1] = 0;
    }
  }
  cout << res;
}
