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

inline bool isK(const int &a, const int &b, const int &c) {
  if (a == b or a == c or b == c) return 0;
  if ((a > b && c > b) or (a < b && c < b)) return 1;
  return 0;
}

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 (isK(v[i], v[i + 1], v[i + 2])) ans++;
  }
  cout << ans << endl;
  getchar();
}