Professor Yeonjun had one rule: keep your distance from students.

He was known as the coldest mathematics professor at the university. Strict, disciplined, and impossible to impress. Most students were too intimidated to speak in his class.

Then there was Rin.

She was always late, always asking difficult questions, and never accepted an answer without arguing. To Yeonjun, she was nothing but a distraction. Every lecture somehow turned into a debate because of her, and by the end of each class, she had tested his patience more than anyone else.

"If you spent half as much time studying as you do arguing," he said one day, "you'd probably have the highest grade in this class."

Rin only smiled.

"Maybe I just like making you think, Professor."

Her confidence irritated him.

Weeks passed, yet Rin didn't change. She kept challenging him, staying after class to ask questions, and refusing to be intimidated by his cold attitude.

But slowly, something changed.

Yeonjun began to notice that she wasn't trying to annoy him—she genuinely loved mathematics. She stayed in the library until midnight, helped struggling classmates without expecting anything in return, and never gave up on solving a problem, no matter how difficult it was.

The more he learned about her, the harder it became to dislike her.

One evening, while grading assignments, he caught himself smiling at one of Rin's comments written in the margin.

He immediately frowned.

What am I doing?

He closed the notebook and rubbed his forehead.

Somewhere along the way, the student he couldn't stand had become the first person he looked for when he walked into the classroom.

That realization unsettled him.

Knowing their roles as professor and student, he buried those feelings and became even more distant, determined not to cross a line that should never be crossed.

Rin couldn't understand why the professor who had finally started looking at her differently had suddenly become colder than ever.

Neither of them knew that some equations couldn't be solved until the timing was finally right.*