IF calculation with aggregate functions

A   B      UA
1   0    Negative
1   1    Negative
1   1    Positive
2   5    Negative
2   2    Positive

I want to calculate %UA Negative such that when A = B then count all the Negatives in the UA column and divide that by total number of results where A = B. So for A=1 and B=1, %UA Negative = 1/2 = 0.5

I tried:

 IF [A] = [B] THEN
     SUM(IF[UA] = 'Negative' THEN 1 ElSE 0 END)/COUNT([UA]) END

but I'm getting the error:

Cannot mix aggregate and non-aggregate comparisons or results in 'IF' expressions