# coding=UTF-8:

n = int(input())
s = list(map(int, input().split()))
count = 0
for i in range(n-2):
    kadomatsu = [s[i], s[i+1], s[i+2]]
    
    if s[i] == s[i+1] or s[i+1] == s[i+2]:
        kadomatsu.clear()
    elif min(kadomatsu) == s[i+1] and kadomatsu[0] != kadomatsu[2]:
        count += 1
    elif max(kadomatsu) == s[i+1] and kadomatsu[0] != kadomatsu[2]:
        count += 1
        kadomatsu.clear()
    else:
        kadomatsu.clear()
print(count)