merge two columns into one

I have a speadsheet with column A and B look like

A     B
0     1
1     0
2     0
0     3

Then I want to create a column C

c
1
1
2
3

That is, the value of column C is the value of A if A is nonempty and the value of B if A is empty. Please advise.

Answer 1

[madhan ravi]

A + B
%or
sum([A, B],2)

[alpedhuez]

Let me work on it.