n = int(input())
a = list(map(int,input().split()))
ans = 0
for i in range(n-2):
	r = set([a[i],a[i+1],a[i+2]])
	if len(r) == 3:
		g = [a[i], a[i+1], a[i+2]]
		g.sort()
		if g[1] == a[i] or g[1] == a[i+2]:
			ans += 1
print(ans)