Problem - Longest Flight Route

You have $n$ cities and $m$ flight routes. Each route is a one-way flight from city $a$ to city $b$.

Find a route from city $1$ to city $n$ that visits the maximum number of cities. Print the maximum number of cities on the route, followed by the cities in order.

If there is no route from $1$ to $n$, print "IMPOSSIBLE".

Example:

• Input: $n = 5$, flights: $1 \to 2$, $2 \to 3$, $3 \to 4$, $1 \to 4$, $4 \to 5$
• Output: $5$ and route $1 \to 2 \to 3 \to 4 \to 5$ (visits all $5$ cities)