#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
  int n, t[3], ans = 0;
  cin >> n;
  vector<int> a(n);
  for (auto& e : a) cin >> e;
  for (int i = 0; i < n - 2; i++) {
    t[0] = a[i];
    t[1] = a[i + 1];
    t[2] = a[i + 2];
    sort(t, t + 3);
    if ((a[i + 1] == t[0] || a[i + 1] == t[2]) && t[0] != t[1] && t[1] != t[2]) {
      ans++;
    }
  }
  cout << ans << endl;
}