import java.util.*;
import java.math.*;
import java.io.*;

public class Main {
	public static void main(String[] args) {
		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 count = 0;
		for(int i = 0; i < N-2; i++) {
			int tmp[] = new int[3];
			tmp[0] = A[i];
			tmp[1] = A[i+1];
			tmp[2] = A[i+2];
			Arrays.sort(tmp);
			if((tmp[1] == A[i] || tmp[1] == A[i+2]) && (tmp[0] != tmp[1] && tmp[1] != tmp[2])) count++;
		}
		System.out.println(count);
	}
}