You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named (2R)-2-[3-(7-hydroxy-4-methyl-2-oxochromen-3-yl)propanoylamino]-4-methylsulfanylbutanoic acid by IUPAC nomenclature, provide its molecular formula.
user
C18H21NO6S
assistant