How can I avoid null values from a 2 columns that I am trying to find the difference for?

I have 2 columns in which I am looking to find the difference for. Since some values of each column contain a null they are not reflected accurately

for example

Column 1 Column2 Diff

2 1 1

4 NULL NULL (should be 4)

NULL 5 NULL (should be -5)

I have the formulas associated with getting values in Column 1 and Column 2 within saved formulas as "KPI(.......)".

My initial idea was to substitute NULL as 0, but when I surround KPI(...) with a CASE WHEN it fails saying that CASE is not supported with INT or STRING

Any ideas?