N = int(input())

A = list(map(int, input().split()))

num_A = len(A)

kadomatsu = 0

for i in range(num_A - 2):
    pair = [A[i], A[i+1], A[i+2]]
    pair.sort()
    if (pair[0] < pair[1] and pair[1] < pair[2]):
        if (A[i+1] == pair[0] or A[i+1] == pair[2]):
            kadomatsu += 1

print(kadomatsu)