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In order not to spoil Cirno’s song, you need to sing along to her, hitting every note and doing it perfectly until the end of the music. In order for Boyfriend to sing to the beat, you need to follow the arrows running on the screen, and press the corresponding keys when they coincide with the fixed identical symbols from Boyfriend’s side. Do not be distracted by the girl on the left, because then you can easily miss the keys and you will have to start singing from the beginning.