Help please.

There is this manga i was reading i forgot about but i want to go back to it. It was about this main character that slept around a lot and was not interested in falling in love was in college. One day this emo-y boy with colorful hair asked him for help on a math puzzle that spelled something out. He runs into the boy on a train, gives the boy a stuffed animal. The boy plays some game on his phone he's embarrassed for anyone to know about it. That's about all I remember.