One of the best, if not the best, Korean dramas I have seen. It had me hooked from the first episode. The plotline, the story, the acting... It was all great. Although it did drag at some points, it eventually picked up its pace and continued with the story. Anyway, I loved everything about it, even the Mathematical references they made every now and then. I think this is one of my personal favourites, and I'd love to recommend it to anyone interested in watching it.