def main():
    N = int(input())
    A = tuple(map(int, input().split()))

    ans = 0
    for i in range(N - 2):
        bamboo = set([A[i], A[i+1], A[i+2]])
        if len(bamboo) < 3:
            continue
        max_ = max(bamboo)
        min_ = min(bamboo)
        if (A[i+1] == min_ or A[i+1] == max_):
            ans += 1

    print(ans)

main()