#include <bits/stdc++.h>
#define rep(i, ss, ee) for (int i = ss; i < ee; ++i)
using namespace std;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int N, ans = 0;
  cin >> N;
  vector<int> v(N);
  rep(i, 0, N) { cin >> v[i]; }
  rep(i, 0, N - 2) {
    if (v[i] == v[i + 1] or v[i] == v[i + 2] or v[i + 1] == v[i + 2]) continue;
    if (v[i] > v[i + 1] && v[i + 2] > v[i + 1]) ans++;
    if (v[i] < v[i + 1] && v[i + 2] < v[i + 1]) ans++;    
  }
  cout << ans << endl;
  getchar();
}