sql server - SSRS: How to SUM row data based on condition -

i have table of structure:-

row1:- not inside details group.

row 2:- dynamically populating row. sequence as:-

a x

a1 1

a2 2

b y

b1 30

b2 40

i need calculate value of y(which value of b1+b2) , x(which value of a1+a2)

also in row 1 need display x+y.

please note:- not sure number of rows can there in either or b. y can b1+b2+b3.... on.

also , b being populated same dataset.

it's bit unclear how data set can't give answer summing based on condition you'd want like:

=sum(iif(fields!a.value = "x", fields!unknown?.value, 0)) 

this sum of unknown field when field a = x.

if isn't looking for, try adding field names , relate them values.