#include <bits/stdc++.h>
using namespace std;

bool check(int a, int b, int c) {
  return (a > b && c > b && a != c) || (a < b && c < b && a != c);
}

int main() {
  int n, ans = 0;
  cin >> n;
  int a[n];
  for (auto& e : a) cin >> e;
  for (int i = 0; i < n - 2; i++) {
    if (check(a[i], a[i + 1], a[i + 2])) ans++;
  }
  cout << ans << endl;
  return 0;
}