2509. What character can be added to both of these characters to give other characters? 止_ -> _同

止  SHI, to(meru), to(maru) - stop

同  DOU, TOU, ona(ji) - same

Add 月:止+月 月+同   

月  GETSU, GATSU, tsuki - moon, month

肯  KOU - agree, consent

胴  DOU - torso, trunk