"Love is the Number of Keys" (also known as Koi wa Hikagaku) is a Japanese adult visual novel developed by Lump of Sugar and released on January 27, 2017. It was later adapted into an adult anime OVA series, with the first episode premiering on July 28, 2017. The story revolves around a young man who discovers a mysterious piano in his new dormitory, leading to a series of romantic and supernatural encounters. The protagonist, Kazuya, moves into a new dormitory and stumbles upon an old, seemingly abandoned piano. When he plays it, he encounters Sera, a ghostly girl who appears only when the piano is played. Sera claims to have died a long time ago and is now bound to the piano. As Kazuya interacts with Sera and other dormitory residents, he uncovers the mysteries surrounding her past and the piano, while also navigating romantic relationships with the living girls in the dorm.
