#include <stdio.h>

int iskado(int a, int b, int c) {
	if (a > b && b > c) return -1;
	else if (a < b && b < c) return -1;
	else if (a == b || b == c || c == a) return -1;
	else return 1;
}

int main() {
	int n, a[100], cnt = 0, i;

	scanf("%d", &n);
	for (i = 0; i < n; i++) {
		scanf("%d", &a[i]);
	}
	for (i = 0; i < n - 2; i++) {
		if (iskado(a[i], a[i + 1], a[i + 2]) == 1) cnt++;
	}

	printf("%d\n", cnt);
	return 0;
}