Nested data validation?

Geniuses-

I'm trying to figure a to enforce dual data validation on a single cell. That is, I need to restrict the user to entering only a decimal value, only if a particular other cell (say A2) is blank. To put it another way, if A2 is blank, the user can enter a decimal value, but if A2 is not blank, the user cannot enter anything.

I can use Data Validation to enforce either the decimal restriction or the ISBLANK, but I'm not sure how to make them work together. Is anyone able to make a suggestion?

Thanks in advance,
Joe in MN