You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-[(2,3-difluorophenyl)methyl]-2-[1-[(2-methoxyphenyl)methyl]-3-oxopiperazin-2-yl]acetamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H23F2N3O3
assistant