import java.util.*;

class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        int[] p = new int[N];
        
        for(int i = 0; i < N; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int count = 0;
        for(int i = 1; i < N-1; i++) {
            if(p[i-1] < p[i] && p[i] < p[i+1] || p[i+1] < p[i] && p[i] < p[i-1]) {
                count++;
            }
        }
        
        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