Find the probability of reuniting with a lost first love

Seonwoohyun dropped his phone in the sewer and got a new one. So, that day was a particularly brutal day for bullies. At 2 a.m., a message arrived from an unknown number. […Are you sleeping?] The message didn't stop there. After getting a new smartphone, Seonwoohyun started getting messages every dawn, as if telling him it was time to go to bed. The ex-boyfriend's messages were explicit, dirty, and increasingly cryptic. They included threats, of course. He didn't hesitate to call if he got the wrong number, and every dawn, he beeped like an alarm clock. The incident occurred on the first day of school, when the group members were exchanging numbers for a liberal arts group project. "Um...