You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(10aS)-2-(4-ethylphenyl)-3-sulfanylidene-10,10a-dihydro-5H-imidazo[1,5-b]isoquinolin-1-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C19H18N2OS
assistant