Set DBNull value to Zero (0) in DataTable Compute function in ASP.Net

How to set default value 0 in compute function.

I am fetching Arrears from the database. I want to set default value 0 for the following statement In case of Arrears are null

int Arr = Convert.ToInt32(dtfinal.Compute("sum(Arrears)", ""));

how to do it???

code is

int Fee = Convert.ToInt32(dtfinal.Compute("sum(Fees)", ""));
int Other = Convert.ToInt32(dtfinal.Compute("sum(Others)", ""));
int Reci = Convert.ToInt32(dtfinal.Compute("sum(Recievable)", ""));
//int Arr = Convert.ToInt32(dtfinal.Compute("sum(Arrears)", ""));
int Net = Convert.ToInt32(dtfinal.Compute("sum(NetBal)", ""));

i am displaying data in datagridview by using the following code

con = new SqlDbConnect();
con.SqlQuery(@"select SPic,fg.Year,fg.Month,IssueDate,DueDate,ParentID,FName,FPhone,fg.AdmissionNo,SName,ClassName,SectionName,Fees,sum(Price) 'Others',ISNULL(Fees,0) + ISNULL(sum(Price),0) 'Recievable',RemBal 'Arrears'
                    ,isnull(( ISNULL(Fees,0) + ISNULL(sum(Price),0)),0) + isnull((RemBal),0) 'NetBal'
                    from tblFeesGenerate as fg left join tblFeesCollection as fc on fg.AdmissionNo=fc.AdmissionNo inner join tblStdReg as sr on fg.AdmissionNo=sr.AdmissionNo
                    inner join tblDefClass as dc on fg.ClassID=dc.ClassID inner join tblDefSection as ds on fg.SectionID=ds.SectionID
                    where fc.AdmissionNo is null or fc.AdmissionNo is not null and
                    fg.Year = @Year and fg.Month=@Month and fg.YearID=@YId and
                    fc.AdmissionNo not IN(SELECT AdmissionNo FROM tblFeesCollection WHERE FeesMonth = @Month AND Paid >= 0 )
                    and FeeID IN (SELECT MAX(FeeID) FROM tblFeesCollection group by AdmissionNo)
                    Group by SPic,ParentID,fg.AdmissionNo,fg.Month,RemBal,SName,FName,fg.Year,fg.ClassID,ClassName,fg.SectionID,SectionName,FPhone,Fees,IssueDate,DueDate order by ParentID desc;");

con.Cmd.Parameters.Add(new SqlParameter("@Year", this.cmbPYear.SelectedItem.ToString()));
con.Cmd.Parameters.Add(new SqlParameter("@Month", this.cmbMonth.SelectedItem.ToString()));
con.Cmd.Parameters.Add(new SqlParameter("@YId", this.cmbYear.SelectedValue.ToString()));