import java.util.Scanner;
	

public class Main{
	
	
	public static void main(String args[])throws Exception{
		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++){
			if(isKadomatsu(A[i],A[i+1],A[i+2])){
				ans++;
			}
		}
		System.out.println(ans);
		
	}

	static boolean isKadomatsu(int i, int j, int k) {
		if(i!=j&&j!=k&&k!=i){

			if((i<j&&j>k)||(i>j&&j<k)){
				return true;
			}
		}
		return false;
	}
	

}