import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner koko = new Scanner(System.in);
       int n = koko.nextInt();
       int[] a = new int[n];
       for(int i = 0; i<n; i++){
           a[i] = koko.nextInt();
       }
       int kado = 0;
       for(int i = 1; i<n-1; i++){
           if(a[i-1]<a[i]&&a[i]>a[i+1]&&a[i-1]!=a[i]&&a[i]!=a[i+1]&&a[i-1]!=a[i+1]){
               kado = kado + 1;
           }else if(a[i-1]>a[i]&&a[i]<a[i+1]&&a[i-1]!=a[i]&&a[i]!=a[i+1]&&a[i-1]!=a[i+1]){
               kado = kado + 1;
           }
       }
       System.out.println(kado);
    }
}