import collections

tmp = input()
N = [int(i) for i in input().strip().split(' ')]

c = 0

for i in range(len(N) - 2):
  d = sorted(N[i:i+3])[1]
  if N[i+1] != d:
    if len(collections.Counter(N[i:i+3])) == 3:
      c += 1

print('{}'.format(c))