You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-[4-amino-5-[(E)-3-(4-chlorophenyl)-3-methylbut-1-enyl]-6-methylpyrimidin-2-yl]acetamide The above is the IUPAC name of a molecule. Write its molecular formula.
user
C18H21ClN4O
assistant