a=int(input())
b=[int(i) for i in input().split()]
c=0
for i in range(2,a):
    if (b[i-1]>b[i]) and (b[i-1]>b[i-2]) or (b[i-1]<b[i]) and (b[i-1]<b[i-2]):
        c+=1
print(c)