You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
6,8,9-tritert-butyl-2-[(4-methoxyphenyl)methyl]-1,3,4,8-tetrahydropyrazino[1,2-c]pyrimidine is the IUPAC name of a molecule. Please give its molecular formula.
user
C27H43N3O
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