N = int(raw_input())
A = map(int, raw_input().split())
cnt = 0
for i in range(1, N-1):
	now = [A[i-1], A[i], A[i+1]]
	now.sort()
	if now[1] != A[i] and now[0] != now[1] and now[1] != now[2] and now[2] != now[0]:
		cnt += 1
print cnt