N = int(raw_input())
a = map(int,raw_input().split())
c = 0
for i in range(len(a)-2):
	if a[i] != a[i+1] and a[i] != a[i+2] and a[i+1] != a[i+2]:
		if (a[i] < a[i+1] and a[i] > a[i+2]) or (a[i] > a[i+1] and a[i] < a[i+2]) or (a[i+2] < a[i+1] and a[i+2] > a[i]) or (a[i+2] > a[i+1] and a[i+2] < a[i]):
			c += 1
print c