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