You almost had it in 1 "instruction". There's another way to get the min/max values.

You're forced to use shorter even with the modern, unless you accept IIII for 4. But the other said no 3 consecutive symbols.

In my opinion, the author should have made it receive ints or make tests with longs.
I'm not saying your solution is bad, it pretty much is like mine but I was forced to create a new Enumerable.Range function that accepted long.

I suggest adding test inputs that are in long range instead of just int.

You're converting to int and back to long, won't work on bigger values