N=int (input())
A=[int(i) for i in input(). split ()]
C=0
i=0
while i<N-2:
    if A[i]!=A[i+1] and A[i]!=A[i+2]:
        if A[i]<A[i+1] and A[i+1]>A[i+2]:
            C+=1
        elif A[i]>A[i+1] and A[i+1]<A[i+2]:
            C+=1
    i+=1
print (C)