#include <iostream>
#include <vector>

using namespace std;

int main() {
  int N;
  cin >> N;
  vector<int> v(N);
  
  for (int i=0; i<N; i++) {
    cin >> v[i];
  }

  int cnt=0;
  for (int i=1; i<N-1; i++) {
    if (v[i-1]!=v[i+1] && ((v[i]>v[i-1] && v[i]>v[i+1]) || (v[i]<v[i-1] && v[i]<v[i+1]))) cnt++;
  }

  cout << cnt << endl;
  return 0;
}