From Tapas:
From the back of the wedding hall, Yuri watches his first love say her vows. His step-sister is marrying some random guy. Next to him, his favorite high school teacher quietly sobs, drunk. Yuri hasn’t seen Mr. Lee in years, so what exactly is he doing here? Suspicious, heartbroken, and determined to uncover a scandal, Yuri confronts him. But, one confused kiss later, Yuri realizes it’s the groom’s marriage Mr. Lee is mourning, not the bride’s.

Shares the same universe with The Shape of Your Love

Original Webtoon:
R15: Ridibooks, MrBlue, Naver Series, BookCube, Postype
R19: Ridibooks, MrBlue, Naver Series, BookCube, Lezhin, Postype

Official Translations:
R15: English, French, German, Spanish
R19: English, German
French: Le Bontoon, Delitoon