You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-(3-methoxyphenyl)-N-[(6-phenyl-[1,2,4]triazolo[4,3-b]pyridazin-3-yl)methyl]acetamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H19N5O2
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