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