how to add column values in rdlc

Question

i have a report in which i want to add column value.like

Name          vehicles 

sa               23

wa               22 

xc               10

 Total:           55

i have created one row outside the group, and the textbox which is below vehicle column i put an expression like

=Sum(Fields!vehicles.Value)

But its gives error,on the other end if i put Count instead of Sum it return 3,what is the problem is here?