2664. What character can be added to both of these characters to give other characters? 其_ -> _長

其  shi, so - that

長  CHOU, naga(i) - long

Add 月:其+月 月+長   

月  GETSU, GATSU, tsuki - moon, month

期  KI, GO - time, period, term

脹  CHOU - swell