Wrapping with path mapper

I have a data tree that goes more or less like this:

{19;0},{19;1},{19;2}.....{19;123}

I want to use path mapper to shift the path of each one so that it would look something like this:

{19;123},{19;0},{19;1}...{19;122}.

To do this, I used path mapper using theses masks:

source: {a;b}

target{a;b-1}

Obviously, the "b-1" is the root of the problem, but I'm wondering if there is an operation in path mapper that would allow me to wrap the numbers so that when it goes past zero, it would flip around. Any ideas?

Thanks