[SQL] To sum column values in SQL Server

In the query shown here, I want to sum the columns of freightCharge and LoadingandUnloadingcharges and add the result as new column totalcost.

I have tried this code and I'm unable to get the desired result:

SELECT DISTINCT
GRN.GoodsReceivedNoteNo,
CASE
WHEN COUNT(GRND.GoodsReceivedNoteID) OVER (PARTITION BY GRND.GoodsReceivedNoteID) > 1
THEN ISNULL((GRND.FreightCharges), 0.00)
ELSE ISNULL((OEC.FreightCharges), 0.00)
END FreightCharge,
CASE
WHEN COUNT(GRND.GoodsReceivedNoteID) OVER (PARTITION BY GRND.GoodsReceivedNoteID) > 1
THEN ISNULL((GRND.LoadingCost + GRND.UnloadingCost), 0.00)
ELSE ISNULL((OEC.LoadingCost + OEC.UnloadingCost), 0.00)
END LoadingandUnloadingcharges
FROM
GoodsReceivedNoteDetail GRND
LEFT OUTER JOIN
GoodsReceivedNote GRN ON GRN.GoodsReceivedNoteID = GRND.GoodsReceivedNoteID
LEFT OUTER JOIN
OtherExpenseCost OEC ON OEC.GoodsReceivedNoteID = GRN.GoodsReceivedNoteID

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