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

    ans = 0
    for i in range(N - 2):
        max_ = max(A[i], A[i+1], A[i+2])
        min_ = min(A[i], A[i+1], A[i+2])
        if (A[i+1] == min_ or A[i+1] == max_) and A[i] != A[i+2]:
            ans += 1

    print(ans)

main()