Highlight value in one column if its negative value is in another column.

I have two columns of dollar values - one is an invoice amount, and the other is the amount paid. I need to be able to see which invoice dollar amounts have a corresponding negative value. For my conditional formatting, I used =COUNTIF(($B2:$B400),-($A2:$A400)) but it works inconsistently. In other words, it highlights $161.47 because there is a $-161.47, but it doesn't highlight $239.72 even though there's a $-239.72.

Can you help me understand this? I don't get what the a1>0 means. What is a1 referencing?