#include <stdio.h>
#include <stdlib.h>

int main()
{
	int N, * A, count = 0;

	if (scanf("%d", &N) != 1) {
		return 1;
	}

	A = (int*)malloc(sizeof(int) * N);

	for (int i = 0; i < N; i++) {
		if (A != NULL) {
			if (scanf("%d", &A[i]) != 1) {
				return 1;
			}
		}
	}

	for (int i = 0; i < N - 2; i++) {
		if (A != NULL) {
			if (A[i] < A[i + 1] && A[i] != A[i + 2] && A[i + 1] > A[i + 2] ||
				A[i] > A[i + 1] && A[i] != A[i + 2] && A[i + 1] < A[i + 2]) {
				count++;
			}
		}
	}

	printf("%d", count);

	free(A);

	return 0;
}