import sys
input=lambda: sys.stdin.readline().rstrip()
n=int(input())
A=[int(i) for i in input().split()]
ans=0
for i in range(1,n-1):
  a,b,c=A[i-1:i+2]
  if a!=b and b!=c and c!=a:
    if b<a<c or c<a<b:
      ans+=1
    elif b<c<a or a<c<b:
      ans+=1
print(ans)