def main():
    num = int(input())
    values = list(map(int, input().split()))
    ans = 0

    for i in range(num - 2):
        tmp_list = [values[i], values[i+1], values[i+2]]
        sorted_list = sorted(tmp_list)
        if len(set(tmp_list)) != 3:
            continue
        if sorted_list[1] == tmp_list[0] or sorted_list[1] == tmp_list[2]:
            ans += 1

    print(ans)

if __name__ == '__main__':
    main()