Hopcorn+

Korn was hit at the head during a robbery, when he wake up, he lost his memory over what had happen before. Now he has to write down everything before he forgets them. However, Korn always feels that there is something happened to someone during the robbery and he has to find out before it's too late. Time passes and Korn is still unable to recall who is that "someone". One day, Korn was told about a haunted 13 floor near his house, and coincidentally he appears at that floor for some reason. Is it "someone" that draws Korn's attention to the 13th floor? What happened to that "someone" during the robbery?