to do calculation of the data retirieved from csv file

hi
i have some values in the csv file, i retrieve it using getCSVValueAt() then i store it in some variable
then i performed addition on the values retrieved using getcsv, but i got some exception

V : ------------------------
16:49:41:469 SERV : CSV Value at 0 , 1 is 3913.20
16:49:41:583 PLAYBACK : Message -> 3913.20
16:49:41:590 SERV : CSV Value at 1 , 1 is 2085.60
16:49:41:702 PLAYBACK : Message -> 2085.60
16:49:41:709 PLAYBACK : Exception type and value are <class exceptions.TypeError at 990232> - __sub__ nor __rsub__ defined for these operands
16:49:41:717 PLAYBACK : Reports were generated

regards
ketan