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