SSRS Expression

Hi i am wanting to create an expression in SSRS to return "2017 - 2018" based on the startdate field and enddate field. I cannot get it to work any ideas?

=(Format(First(Fields!S_StartDate.Value, "MemberDetails"), "yyyy") + " - " + (Format(First(Fields!S_EndDate.Value, "MemberDetails"), "yyyy")

Thanks