n=int(input())
a=list(map(int,input().split()))
ans=0
for i in range(n-2):
    if a[i]!=a[i+1] and a[i]!=a[i+2] and a[i+1]!=a[i+2] and ((a[i]<a[i+1] and a[i+1]>a[i+2]) or (a[i]>a[i+1] and a[i+1]<a[i+2])):
        ans+=1
print(ans)