n=int(input())
lsm=list(map(int,input().split()))
cnt=0

for i in range(len(lsm)-2):
    ls1=[lsm[i],lsm[i+1],lsm[i+2]]
    lsc=lsm[i+1]
    ls1.sort()
    if lsc!=ls1[1] and ls1[0]!=ls1[1] and ls1[1]!=ls1[2]:
        cnt+=1
print(cnt)