I can't understand the difference between count and sum

I'm trying to get the number of rows for a specific condition of my report and always I get 0 using count or sum

```
columnA columnB
0       my string  
0       none
0       my string  
0       none
1       none

```

=`IIF((Fields!columnA.Value = "0" and Fields!columnB.Value = "none"), SUM(Fields!columnA .Value),0)`

I want to get 2 as a result
I can't see my error