_ = input()
a = list(map(int, input().split()))

count = 0

for x, y, z in zip(a[:-2], a[1:-1], a[2:]):
    trio = sorted([x, y, z])

    if (trio[1] == x or trio[1] == z) and trio[0] != trio[1] != trio[2]:
        count += 1

print(count)