You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-[1-(2,4-dichlorophenyl)ethyl]-2-(1,4-dioxo-3H-phthalazin-2-yl)acetamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H15Cl2N3O3
assistant