from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
input = sys.stdin.readline

N = int(input())
A = list(map(int,input().split()))

ans = 0
for i in range(1,N-1):

    ans += (A[i]>A[i-1])&(A[i]>A[i+1])&(A[i-1]!=A[i+1])
    ans += (A[i]<A[i-1])&(A[i]<A[i+1])&(A[i-1]!=A[i+1])

print(ans)