You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
tert-butyl (2R,7R)-7-acetyloxy-2-(2-bromo-1H-indol-3-yl)-4-azaspiro[2.4]heptane-4-carboxylate The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H25BrN2O4
assistant