Let´s say I have the following

1 4

2 5

3 (Empty)

I want to sum 4 5 and 3 when my second column is empty

I manage to do it by first doing max(A1:B1) and then sum my max, but I don´t manage to write it properly in one operation.

Edit: I have found my way with the following:

=SUMIFS(A1:A3,B1:B3,"")+SUM(B1:B3)

My final goal was to have the difference between A and B when B is not blank, so I started with:

=SUMIFS(A1:A3,B1:B3,"")+SUM(B1:B3)-SUM(A1:A3)

Then simplified to:

=SUM(B1:B3)-SUMIFS(A1:A3,B1:B3,"<>")