#coding=UTF-8

#真面目に歩くだけかな
N=int(input())
mojir=input()
hyo=mojir.split(' ')
A=[int(mono) for mono in hyo]

ans=0
for idx in range(0,N-2,1):
    left=A[idx]
    middle=A[idx+1]
    right=A[idx+2]
    if (left < middle and middle > right) or \
       ( left > middle and middle < right):#傾向と対策:門松列の定義より
        if len(set([left,middle,right]))==3:#3つバラバラ
            ans=ans+1

print(ans)