I need someone to help me please.

Example:
cell a2 = 5,000
cell a3 = 10,000
cell a4 = 90
cell b2 = "blank"
cell b3 = 5,000
cell b4 = 90

i am writing a SUMIF problem =sumif(a4:b4,90,a3:b3)

but what i need is the sum of cells a3 if a2 is blank, if cell a2 has a
number then i need that number. in other words the total of this formula
needs to = 10,000 not 15,000.

Can anyone help me?

Susan