Countif multiple conditions

Hi I want to be able to look at two columns and set a criteria to count. I want to look at column A and if its blank then look at column B and if it has a value of more than 0 then count.

A B
1 1.00
2 Yes 4.78
3
4 5.00
5 Yes 4.89
6 11.99

So this example would count 3

If that all makes sense.

David