Title?

After reading a manga I remembered a different BL that had like a backstory of a boy who was at the amusement park but instead of going along with his classmates he met an older guy reading I think and at some point the boy had to go to the toilet, but the older guy went with someone else and left a balloon with a note (his number) at the bench but right when the boy got there it flew away and I’m not sure but later when they’re both adults they realised that they have already met or something, thanks in advance~