六场会最多能排下几场

六场会的起止时间是 [(1,4),(2,3),(3,5),(0,7),(5,6),(6,8)]，按结束时间从早到晚挑。运行下面这段程序：

def sched(iv, key):
    end = -1
    n = 0
    for s, e in sorted(iv, key=key):
        if s >= end:
            n += 1
            end = e
    return n

print(sched([(1, 4), (2, 3), (3, 5), (0, 7), (5, 6), (6, 8)], lambda t: t[1]))