#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> a;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    a.push_back(x);
  }
  long long ans = 0;
  for (int i = 1; i < n - 1; i++) {
    if (a[i] < a[i - 1] and a[i] < a[i + 1])
      ans++;
    else if (a[i] > a[i - 1] and a[i] > a[i + 1])
      ans++;
  }
  cout << ans;
}
