import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();

		int l = sc.nextInt();
		int m = sc.nextInt();
		int r = 0;
		int ans = 0;

		for(int i=0;i<n-2;i++){
			r = sc.nextInt();
			if(l<=m&&m<=r||l>=m&&m>=r)ans++;
			l=m;
			m=r;
		}

		// 出力
		System.out.println(ans);
	}
}
/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