import java.util.*;

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