How to calculate the sum gridview with paging?

protected void GetPreviousTotal()

{

DataSet dset = get data from database;
OR 
DataSet dset = get data from viewstate // for this you need to store your dataset in viewstate when u get from database for the first time

for (int i = 0; i < dset.Tables[0].Rows.Count; i++)  
{
    if (i < grdBISentry.PageIndex * grdBISentry.PageSize)
    {
        total += Convert.ToDouble(dset.Tables[0].Rows[i]["COLUMN_NAME"].ToString());
    }
    else
        break;
}

}