yào / jiě / jué / zhè / dào / tí / wǒ / men / xiān / gēn / jù / tiáo / jiàn / xiě / chū / dì / tuī / gōng / shì要解决这道题,我们先根据条件写出递推公式。To solve this problem, we first write a recurrence formula based on the given conditions.