package m.shin;
import java.util.Scanner;
public class Con116 {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int N = s.nextInt();
		int[] A = new int[N];
		for(int i = 0;i < N;i++){
			A[i] = s.nextInt();
		}
		s.close();

		int count = 0;
		for(int i = 0;i < N -2;i++){
			if((A[i] - A[i+1]) * (A[i+2] - A[i+1]) > 0 && A[i] != A[i+2]){
				count++;
			}
		}
		System.out.println(count);

	}

}