A compound represented by formula (I):wherein R is a methyl group or the like, R1 is a fluorine atom or the like, R2 is a hydrogen atom or a fluorine atom, R3 is a hydrogen atom, R4 is an ethyl group or the like, or a pharmaceutically acceptable salt thereof.由化學式(I)表示之化合物或其藥學上可接受之鹽:其中R係甲基或類似物,R1係氟原子或類似物,R2係氫原子或氟原子,R3係氫原子,R4係乙基或類似物。