finally i got the answer, just want to share it to everyone.
i create formula fields named "sum_amount". then i put this code.
if isnull({GetSaleDetails.SHA_AMOUNT}) then 0
else cdbl({GetSaleDetails.SHA_AMOUNT})
then i go back to summarize get the "sum_amount"
then select "sum" in calculate option. :D
dfolzenlogen is right again. options to calculate depends on the field type. so if field is string you cant see "sum" option.