public class No_116 {
	static int kadomatsu;

	static void countKadomatsu(int[] array, int i) {
		if (i + 1 >= array.length) {
			return;
		}
		int left = array[i - 1];
		int right = array[i + 1];
		int[] newArray = { array[i - 1], array[i], array[i + 1] };
		for (int j = 0; j < newArray.length - 1; j++) {
			for (int k = j + 1; k < newArray.length; k++) {
				if (newArray[j] > newArray[k]) {
					int tmp = newArray[j];
					newArray[j] = newArray[k];
					newArray[k] = tmp;
				}
			}
		}

		if ((newArray[1] == left || newArray[1] == right)
				&& (newArray[0] != newArray[1] && newArray[1] != newArray[2])) {
			kadomatsu++;
		}
		countKadomatsu(array, ++i);
	}

	public static void main(String[] args) {
		java.util.Scanner sc = new java.util.Scanner(System.in);
		int num = sc.nextInt();
		int[] array = new int[num];

		for (int i = 0; i < num; i++) {
			array[i] = sc.nextInt();
		}

		countKadomatsu(array, 1);
		System.out.println(kadomatsu);
		sc.close();

	}

}