n = int(input())
a = [int(x) for x in input().split()]

res = 0
for i in range(n - 2):
  al = a[i]
  ac = a[i + 1]
  ar = a[i + 2]
  if al != ac != ar != al:
    if al < ac > ar or al > ac < ar:
      res += 1
print(res)