how to sum sub report return value?

[burasami]

hai.. i am having one main report and calling two sub report which will return the values. here my problem is i want to add those return value from sub report. while i am adding that value its giving null value

 

example:

i am getting return value Total_A and Total_B from two sub report. this two variable are displayed as text field in group footer.

when i am adding this field into temp variable called Grand_Total. which result null value.. such as (Grand_Total=Total_A + Total_B)

 

is any beware to slove this problem? help me its very urgent.

thanks in advance