You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Can you give the molecular formula of the molecule 2-[(3S)-2',5'-dioxospiro[1,2-dihydroindene-3,4'-imidazolidine]-1'-yl]-N-(2-phenylethyl)acetamide ?
user
C21H21N3O3
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