Problem - Longest Flight Route

You are given a flight network with $n$ cities and $m$ flights. Each flight goes from city $a$ to city $b$. You want to find a route from city $1$ to city $n$ that visits the maximum number of cities. If there is no route from $1$ to $n$, output "IMPOSSIBLE".

Otherwise, output the number of cities in the longest route and the route itself. This is a longest path problem on a DAG, but you are maximizing the number of nodes visited, not edge weights.