# -*- coding: utf-8 -*-

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

counter = 0
for i in range(N-2):
    a, b, c = A[i], A[i+1], A[i+2]
    maximum = max(a, b, c)
    minimum = min(a, b, c)
    if (a!=b and b!=c and c!=a) and (maximum == b or minimum == b):
        counter += 1
print(counter)