#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  int ans = 0;
  for (int i = 0; i < n; i++) cin >> a[i];
  for (int i = 1; i < n - 1; i++)
    if (a[i] < a[i - 1] && a[i] < a[i + 1])
      ans++;
    else if (a[i] > a[i - 1] && a[i] > a[i + 1])
      ans++;
  cout << ans << endl;
  return 0;
}
