N = int(input())
A = list(map(int,input().split()))

cnt=0
for i in range(N-2):
    temp=[A[i],A[i+1],A[i+2]]
    if A[i]<A[i+1]>A[i+2] or A[i]>A[i+1]<A[i+2]:
        if A[i]!=A[i+2]:
            cnt+=1

print(cnt)