N=int(input())
S=list(map(int,input().split()))
c=0
for i in range(len(S)-2):
    if S[i]!=S[i+2]:
        if S[i]<S[i+1]>S[i+2] or S[i]>S[i+1]<S[i+2]:
            c+=1
print(c)