how to convert number in parenthesis to negative sign in datatable in c#?

Below code is only converting positive values to 2 decimals. I also need to convert negatives values that are enclosed in brackets ( ) to 2 decimals.

if (dr["Market Value"] != DBNull.Value)
                {
                    dr["Market Value"] = Convert.ToDecimal(dr["Market     Value"]).ToString("#.##");