import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
        int P[] = new int[N];
        for(int i = 0; i < N; ++i){
            P[i] = sc.nextInt();
        }
        int count = 0;
        for(int i = 0; i < N-2; ++i){
            if(P[i] < P[i+1] && P[i+1] < P[i+2] || P[i] > P[i+1] && P[i+1] > P[i+2]){
             count += 1;   
            }
        }
        System.out.println(count);
	}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz