# -*- coding:utf-8 -*-

if __name__ == "__main__":
	n = input()
	take = map(int,raw_input().split())
	ans = 0
	for i in xrange(1,n-1):
		if (take[i] > take[i-1] and take[i] < take[i+1]) or (take[i] < take[i-1] and take[i] > take[i+1]):
			continue
		if take[i] == take[i+1] or take[i] == take[i-1] or take[i+1] == take[i-1]:
			continue
		ans += 1
	print ans