Alice and Bob's Game Points

Alice and Bob were playing a game where they earn points based on their performance in various challenges. Alice scored 3 points in the first challenge, 5 points in the second challenge, and $x$ points in the third challenge. Bob, on the other hand, scored 4 points in the first challenge, $y$ points in the second challenge, and 6 points in the third challenge.

If Alice scored a total of 20 points and Bob scored a total of 18 points, what is the value of $y$?