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