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