def isKadomatsu(a, b, c):
    l = [a, b, c]
    if len(l) != len(set(l)):
        return False
    sl = sorted(l)
    if sl[1] == a or sl[1] == c:
        return True
    else:
        return False

N = input()
K = list(map(int, input().split()))

res = 0
for i, Ki in enumerate(K):
    if i == 0 or i == len(K) - 1:
        continue
    if isKadomatsu(K[i-1], K[i], K[i+1]):
        res += 1
print(res)