Building an Inversely Proportional Relationship

If the value from null9 gets higher and higher, then ‘math3’ gets lower and lower,
On the contrary, if the value from null9 gets lower and lower, I want to build it as an inverse relationship so that ‘math3’ gets higher and higher.
How should I adjust it?
It’s an inverse relationship, not a direct relationship.

Maybe have a look at Expression CHOP in wiki and op snippets…