# -*- coding:utf-8 -*-
is_kadomatsu = lambda a, b, c: sorted((a, b, c))[1] in [a, c] and a != b and b != c and a != c
if __name__ == '__main__':
    n = int(input())
    an = tuple(map(int, input().split()))
    cnt = 0
    for i in range(len(an)-2):
        cnt += is_kadomatsu(*an[i:i+3])
    print(cnt)