
import java.util.*;

class Main{
    public static void main(String[] args) {
        Scanner stdIn = new Scanner(System.in);
        int n = stdIn.nextInt();
        int[] p = new int[n];
        for(int i = 0; i < n; i++){
            p[i] = stdIn.nextInt();
        }
        int count = 0;
        for(int i = 1; i + 1 < n; i++){
            if((p[i - 1] - p[i]) * (p[i + 1] - p[i]) < 0){
                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