Perfect Number (2012) on Netflix

Plot summary

Kim Seok-go was a promising mathematician in school, absorbed in his studies rather than in friends, and now, in his thirties, he teaches math at a high school. He is solemn and introverted. The brightest part of his day is the morning stop at the cafe where Baek Hwa-sun works, and he buys his lunch there. Her ex-husband beats her and her niece, and the man dies at Hwa-sun's hands. Seok-go hears the struggle from the house next door. He decides to hide what happened and to keep her out of the police's reach, using his talent for mathematics to plan an alibi for her. Jo Min-beom leads the investigation and believes Hwa-sun is guilty. Seok-go and he attended the same high school.