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

counter = 0
for i in range(1, n - 1):
    if ls[i - 1] <= ls[i] <= ls[i + 1]:
        continue
    elif ls[i + 1] <= ls[i] <= ls[i - 1]:
        continue
    elif ls[i - 1] == ls[i] or ls[i] == ls[i + 1] or ls[i - 1] == ls[i + 1]:
        continue
    else:
        counter += 1

print(counter)