Silver Devil King

Yooan Ha, high school senior, failed what were reportedly the "easiest SATs ever" because he filled in the scantron incorrectly. As he stares into the Han River contemplating his future, he is approached by a demon who offers to turn back time for him. But first, he must go to the demon realm and stand in for the demon king. Will Yooan be able to safely return to his own life afterwards?