n = int(input())
a = list(map(int,input().split()))

count = 0

for i in range(len(a) - 2):
	a0,a1,a2 = a[i],a[i+1],a[i+2]
	if a0 == a1 or a0 == a2 or a1 == a2:
		continue
	b = [a0,a1,a2]
	b.sort()
	if b[1] == a0 or b[1] == a2:
		count += 1

print(count)