Passing the sum from sub report to main report

CAn any one tell me how to pass the sumfunction value from sub report to main report

sum function is sum of the number column of subreport

i have 2 subreports

i want to pass the sumfunction value from two subreports
to the main reports where i want to add again the sum functions of the two subreport

eg say the sum functionvalue of subreport1 =12

sum functionvalue of subreport2 =24
then adding two in main report i want a functionvalue3 = sum functionvalue1+sum functionvalue2 =36

So how can i make this report .
thanks
Saurabh