I didn't experience any happy ending at all, more because the 16 years of age gap that the two had, apart from the math and despite to the presence of love in between them, you can't close your eyes on the reality which is one will die sooner than the other and the absence will enormously damage the other one, not to mention knowing that made me sad too, about the story and the idea behind it I'll say that it was good enough to make you watch it till the end but at some point it got a lil bit boring moreover it would've been better if it had less episodes.
Among all other K-dramas that I've watched so far this was so new to me, watching a show based on this kind of relationship made me really sad(cuz of the reason i mentioned earlier) whether they achieve their goal or not.
IDK about u but I gave this score cuz I think the emotions that the female teacher has are extremely powerful and much deeper so the male student won't be able to handle all that in the long term and this is another sad truth.
