Multiplying two columns of same table and storing result to the third column of same table

I m having a table ``` SALES(sno,comp_name,quantity,costperunit,totalcost) ``` After supplying the `costperunit` values, `totalcost` need to be calculated as `"totalcost=quantity*costperunit".` I want to multiply `'quantity'` and `'costperunit'` columns and store the result in `'totalcost'` column of same table. I have tried this: ``` insert into SALES(totalcost) select quantity*costperunit as res from SALES ``` But It failed! Somebody please help me in achieving this.. Thanks in Advance