# coding: utf-8

n = int(input())
a = list(map(int, input().split(" ")))
res = 0
for i in range(n-2):
    k = a[i:i+3]
    if not(k[0] == k[1] or k[1] == k[2] or k[2] == k[0] or k[1] == sorted(k)[1]):
        res += 1
print(res)