import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int [] A = new int [N];
        int c = 0;
        for(int i=0; i<N; i++){
            A[i] = sc.nextInt();
            if(i>=2){
                if( A[i-1] > A[i] && A[i-1] > A[i-2] && A[i] != A[i-2] ){
                    c++;
                }else if(A[i-1] < A[i] && A[i-1] < A[i-2] && A[i] != A[i-2] ){
                    c++;
                }
            }
        }
        System.out.println(c);
    }
}