Expression that gives negative/positive value on a parented Path Offset?

[Defineyourgoal]

Hi, 

I have created 2 nulls that follows a path. I want Null 2 to follow Null 1 with the opposite value.

For example:

The Path Offset for Null 1 is 80.

Null 2, that's parented to Null 1, should give the same value but negative: -80. 

What expression can be used? 

Thanks

[Mylenium · Accepted solution]

Reversing absolute values is as simple as multiplying things by -1. I'm not clear what you mean by "path offset", though. If you are mirroring something or whatever it is you are doing there of course just multiplying the value wouldn't do anything. You need to define the mirror axis/ center. You need to explain more specifically if that is realyl what you want.

Mylenium

[Defineyourgoal]

Thanks Mylenium! It was exactly what I was looking for. The Path Offset was extra info if the expression would be changed specifically for that.