The worlds has collide together (fem/flippy reader x sans)

After young kid named frisk, who broke the barrier and free the monster race. Monsters and humans have come to peace once again, but the barrier shattered accidently opened the portal between their worlds. Where people and creatures are different from each other. They began to welcome each other. Their own adventure and future has begin. Four years later, they begun get used it, they even have schools for monsters, humans, dimensions people or creatures. Here is sans the skeleton who was walking down the streets and accidently bump into green bear. What happens next?
(btw this is x-over or crossover and this art cover is mine)
sans by toby fox
flippy by happy tree friends