def kado(a,b,c):
	s=sorted([a,b,c])
	if b==s[0] or b==s[2]:
		if len({a,b,c})==3:
			return True
	return False

n=int(input())
*a,=map(int,input().split())
print(sum([kado(a[i-2],a[i-1],a[i]) for i in range(2,n)]))