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t = int(input())
sounds = [int(x) for x in input().split()]
sounds_1 = sounds.copy()
result_down = 0
for i in range(1, t - 1):
    if min(sounds_1[i + 1], sounds_1[i - 1]) <= sounds_1[i] <= max(sounds_1[i + 1], sounds_1[i - 1]):
        sounds_1[i] = -10e9
        result_down += 1

result_up = 0
sounds_2 = sounds.copy()
for i in range(1, t - 1):
    if min(sounds_2[i + 1], sounds_2[i - 1]) <= sounds_2[i] <= max(sounds_2[i + 1], sounds_2[i - 1]):
        sounds_2[i] = 10e9
        result_up += 1


print(min(result_down, result_up))