#include <bits/stdc++.h>
#define fastIO (cin.tie(0), cout.tie(0), ios::sync_with_stdio(false))
#define precise(i) fixed << setprecision(i)
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
using namespace std;

int main() {
  fastIO;

  int n;
  cin >> n;
  vector<int> A(n);
  for (auto &a : A)
    cin >> a;

  int cnt = 0;
  for (size_t i = 0; i < A.size() - 2; ++i) {
    int l = A[i], m = A[i + 1], r = A[i + 2];

    if (l != m && m != r && l != r) {
      if ((m < l && l < r) || (r < l && l < m) || (m < r && r < l) ||
          (l < r && r < m)) {
        cnt++;
      }
    }
  }

  cout << cnt << '\n';
}