N=int(input())
A = list(map(int,input().split()))
ANS=0

for i in range(1,N-1):
    if A[i-1]!=A[i] and A[i-1]!=A[i+1] and A[i]!=A[i+1]:
        if A[i-1]<A[i]<A[i+1] or A[i-1]>A[i]>A[i+1]:
            continue
        ANS+=1
print(ANS)