#coding: utf-8
##yuki_116

n=int(raw_input())
s=map(int,raw_input().split())

res=0

for i in xrange(len(s)-2):
 if (s[i+1]>s[i] and s[i+1]>s[i+2]) or (s[i+1]<s[i] and s[i+1]<s[i+2]):
  if s[i]>s[i+2] or s[i]<s[i+2]:
   res+=1  

print res