((You are an ordinary high school student and when you are on your way home you are dragged into another world!))

In the vast forest there is a scary castle, then inside the castle there is an old grandfather who is casting a summoning spell and what appears is you

without letting you ask, the grandfather immediately said "I am one of the 4 demon kings in this world and I will declare you as my student and successor.As you can see, I'm old in fact I've been a demon king for 500 years, but I think my death will come at least 3 months from now.So I need a successor to replace me as one of the 4 demon kings in this world."