Spoilers
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The novel is competed with 120 chapters with also around 20-30 ish extra chapters/side stories (last I remember it’s that much) however the translator team site I was using had the last 2 out of 5 side stories/extra behind a pay wall for over a year now sadly.
As for their relationship, he knows that he’s his first love from almost from the beginning he saw him! He just wants him to get comfortable and let him remember himself as not to overwhelm him and also other details mentioned in the novel. As for how long it takes for both of them to admit to EACH OTHER that they both now, that happens around after the finish from all of their high school final exams when the MCs hair starts to turn white and he tries to hide it. (I believe it was near chapters 80-ish out of 120?) They’re both 18-19 years old by then as mentioned in the novel. After they get together, and even slightly before that, the focus on trying to go to universities that are near each other to be close and after they get together, they meet each others families and it’s adorable! The buildup from friendship to relationship is so natural and you truly believe that they have mutual care for each other. I really recommend reading the novel!
Spoilers
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He knew almost from the begining that he was his first love! He took a bit of extra time confirm 100% that he was his first love but he was already sure. As for the reaction, so that answers the first question.
For his reaction, it was more of a confirmation for both of them that they both found out and just were hesitant to confront the other about knowing the truth. I don’t know how to put it into words as there’s an explanation of why Yoontae puts so much emphasis and has so much priority on meeting this first love of his that he met only a handful of times on the mountain when he was younger and the novel explains how that they two couldn’t come into contact again after the weather basically interfered with their chances of meeting again (Jeongseo left a letter with his name, phone number and address under the tree they meet at however the winter storm burried it under and that’s why Yeontae couldn’t find it and thought that he was left behind. So when they reveal this to each other it’s just quite cathartic and satisfying the way all the pieces fit together!
Here you go!
https://transweaver.com/series/predator-alert/
It’s machine translation so there are some mix ups over the pronounce and sometimes the names but nothing too extreme :D enjoy!

Guys go read the novel it’s so adorable and so good and cute I’m dying out of cuteness overload if anybody wants spoilers, reply here with questions!