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