#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; ++i) cin >> a[i];
  int ans = 0;
  for (int i = 1; i < n - 1; ++i) {
    if (a[i] > a[i - 1] && a[i] > a[i + 1]) ans++;
    if (a[i] < a[i - 1] && a[i] < a[i + 1]) ans++;
  }
  cout << ans;
  return 0;
}
