n = int(input())
a = [int(i) for i in input().split()]
count = 0

for i in range(n - 2):
    tmp = [a[i], a[i + 1], a[i + 2]]
    tmp.sort()

    if a[i] == tmp[1] or a[i + 2] == tmp[1]:
        if a[i] != a[i + 1] and a[i + 1] != a[i + 2] and a[i + 2] != a[i]:
            count += 1

print(count)