You can use following with changes applicable to your data:
=IF(A1+A2>GETPIVOTDATA("Qtty",F1),"Yes","No")
Where A1 and A2 are cells you are referring to and "Qtty" is a variable in Pivot that you are summing in Values ares. You can see Excel Help for GETPIVOTDATA() and understand about "Qtty" & F1 Refrence.