# -*- coding: utf-8 -*-
N = input()
cnt = 0
ref = []
Alist = map(int, raw_input().split())
for i in range(0, len(Alist)-2):
    j = (Alist[i+2] - Alist[i+1]) * (Alist[i+1] - Alist[i])
    if j < 0 and Alist[i+2] != Alist[i]:
        cnt += 1
print cnt