Calculation ordering question

Hi, thanks in advance for the help on this.

I have two measures which are quite complex. that yield results at a row level that look something like this:

I need to calculate a new meaure which is sum(Measure A - Measure B).

But for each row, if Measure A - Measure B is less than zero, I want to set the result to zero, so the desired result would be:

A1: 3 - 2 = 1

A2: 5 - 9 = 0

A3: 7 - 6 = 1

A4: 2 - 3 = 0

Total = 2

How do I instruct power bi to calculate the measures at a row level applying the zero rule first, then aggregate, rather than aggregating then applying the zero rule.

Thanks

Robbie