CSES 1673 High Score - The Reachability Trap

A negative cycle only gives infinite score if it is on some path connecting node $1$ to node N. If the cycle is unreachable or does not lead to N, it does not help. You need to check two things:

$1.$ Is there a negative cycle?

$2.$ Is it reachable starting at $1$ AND can it reach N? Both conditions must hold for the answer to be $-1$. Check reachability using BFS on the original and reversed graphs.