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