N=int(raw_input())
A=map(int,raw_input().split())
ans=0
for i in range(len(A)-2):
    if A[i]<A[i+1] and A[i+1]>A[i+2] and A[i]!=A[i+2]:
        ans+=1
    if A[i]>A[i+1] and A[i+1]<A[i+2] and A[i]!=A[i+2]:
        ans+=1
print ans