You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 2-(9H-fluoren-9-ylmethoxycarbonylamino)-3-methyl-3-(2-pyridin-4-ylethylsulfanyl)butanoic acid by IUPAC nomenclature, provide its molecular formula.
user
C27H28N2O4S
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