N=int(raw_input())
A=map(int,raw_input().split())
ans=0
for i in xrange(N-2):
    a,b,c=A[i],A[i+1],A[i+2]
    if (a!=b and b!=c and c!=a) and not (a<b<c or a>b>c):
        ans+=1
print ans