You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Translate the chemical IUPAC name (3aS,4R,9bR)-2-methyl-4-(4-methylphenyl)-3a-nitro-4,9b-dihydro-1H-chromeno[4,3-d][1,2]oxazole into its molecular formula.
user
C18H18N2O4
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