Display Sum of the members selected in prompts

Summary:

I have report in EPBCS MR, and I have created a prompt for one dimension and multiple values selection. Now in the report if I select 2 values for the prompt the report should show to sum of the data of the both values selected, instead of showing 2 pages one for each prompt value.

Example: If I create prompt on Entity and if I select 2 members in the prompt. The report should show the values of the sum of the both members instead of showing data for each Entity in separate page.