n = int(input())
l = list(map(int,input().split()))

v = 0
for i in range(1,n-1):
  if (l[i-1] < l[i] > l[i+1] or l[i-1] > l[i] < l[i+1]) and l[i-1] != l[i+1]:
      v+=1
print(v)