# -*- coding: utf-8 -*-

n = int(input())
a = list(map(int,input().split()))

kado = 0

for i in range(2,n):
    if a[i-1] < a[i] < a[i-2] or a[i-2] < a[i] < a[i-1] or a[i-1] < a[i-2] < a[i] or  a[i] < a[i-2] < a[i-1]:
        kado += 1

print(kado)