N = int(input())
b = [int(t) for t in input().split(' ')]
cnt = 0
for idx in range(N-2):
    tmp = b[idx:idx+3]
    if (tmp[1] == max(tmp) or tmp[1] == min(tmp)) and all([tmp[i%3] != tmp[(i+1)%3] for i in range(len(tmp))]):
        cnt += 1
print(cnt)