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

int main(void)
{
	int n, *a, i, s = 0;
	scanf("%d", &n);
	a = malloc(sizeof(int)*n);
	for (i = 0; i < n; i++) scanf("%d", &a[i]);
	for (i = 0; i < n - 2; i++) (a[i]<a[i + 1] && a[i + 1]>a[i + 2] || a[i] > a[i + 1] && a[i + 1] < a[i + 2]) && a[i] != a[i + 2] ? s++ : 0;
	printf("%d\n", s);
	return 0;
}