n=int(input())
a=list(map(int,input().split()))
count=0
for i in range(1,n-1):
    if((a[i]>a[i-1] and a[i]>a[i+1]) or (a[i]<a[i-1] and a[i]<a[i+1])):
#既然是折点，那么就是折点附近的两个点大于、小于折点的值
        count = count + 1
print(count)

   嗷嗷，输入输出终于不报错啦！！！鼓掌鼓掌！！！！