#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n + 1];
  for (int i = 1; i <= n; ++i) {
    cin >> a[i];
  }
  int sum = 0;
  for (int i = 2; i < n; ++i) {
    if (a[i] > a[i + 1] && a[i] > a[i - 1]) sum++;
    if (a[i] < a[i + 1] && a[i] < a[i - 1]) sum++;
  }
  cout << sum;
  return 0;
}
