#include <bits/stdc++.h>
using namespace std;
const int maxn = 105;
int a[maxn], n;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  int ans = 0;
  for (int i = 2; i < n; i++) {
    if (a[i] == 0 && a[i - 1] == 1 && a[i + 1] == 1) {
      a[i + 1] = 0;
      ans++;
    }
  }
  printf("%d\n", ans);
  return 0;
}
