#include <bits/stdc++.h>

using namespace std;

int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; ++i) cin >> a[i];

  int ans = 0;
  for (int i = 0; i < n - 2; ++i) {
    int o = a[i], t = a[i + 1], th = a[i + 2];
    if ((o < t && o > th) || (o < th && o > t) || (th < o && th > t) || (th < t && th > o)) {
      ans++;
    }
  }
  cout << ans << '\n';
  return 0;
}