n = int(input())
a_lst = list(map(int, input().split(' ')))

cnt = 0
for i in range(n-2):
    a_0 = a_lst[i]
    a_1 = a_lst[i+1]
    a_2 = a_lst[i+2]

    if a_0 == a_1 and a_1 == a_2:
        continue

    take_lst = [a_0, a_1, a_2]
    if (a_0 != max(take_lst) and a_0 != min(take_lst) ) or (a_2 != max(take_lst) and a_2 != min(take_lst)):
        cnt += 1

print(cnt)