n=int(input())
a=list(map(int,input().split()))
ct=0
for i in range(n-2):
  x=a[i];y=a[i+1];z=a[i+2]
  l=[x,y,z]
  l.sort()
  if x!=y and y!=z and z!=x and (l[1]==x or l[1]==z):
    ct+=1
print(ct)