Get First Value from a group

Hi Guys,

I'm trying to get the First record under Department Group Column RS which is Highlighted in yellow. 

In this case I should be getting 89 RS (Stands for Rooms Sold) 

I have tried this function 

=First(Fields!RS.Value,"Department")

it works on same date Parameter (highlighted in Yellow) but when I change the date from Start Date 06/28/2014 to End Date 06/29/2014 it suppose to add (89+85)=174 RS, however I get 85 in the department group which is the First record

I was hoping that you guys can point me in the right direction. 

Thank you for your help

Codernater