How do you convert an input parameter string into a number

Hello...

I'm trying to create a selection formula on a hierarchically grouped report where the input is in the form of "1,2,3,4,23,45" as string. How do I create a statement like:

{view_assets.AssetId} in [1, 2, 5]

The problem is that after the "in" it's looking for a number. How do I convert the input string to a number that will work with "in"?

Thanks in advance,

Donald