#include <bits/stdc++.h>
using namespace std;
const int N = 1e4 + 5;
int n, a[N];
int main() {
  ios ::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  if (fopen("main.in", "r")) freopen("main.in", "r", stdin);
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  int cnt = 0;
  for (int i = 2; i < n; i++)
    if (a[i - 1] == 1 && a[i + 1] == 1 && a[i] == 0) a[i + 1] = 0, cnt++;
  cout << cnt;
  return 0;
}
