Gridview Column Addition

Question

Hi
I have grid as follows

Sr No	Min Value	Max Value
1	1	500
2	501	700

I have two text box 1.Min Value 2.Max Value
and one add button
Now i want that first i enter min value as 1 and max as 500
when next time i enter the values Min value should automatically displayed as 501 as shown above
(i.e last Max Value + 1)
 how to do it im using following code to add values to grid

protected void BtnAdd_Click(object sender, EventArgs e)

    {

        DataTable obj = GetEmptyTable();

        foreach (GridViewRow gvRow in this.gv.Rows)

        {

            obj.Rows.Add(

                this.gv.DataKeys[gvRow.RowIndex].Values["MinValue"], 

                this.gv.DataKeys[gvRow.RowIndex].Values["MaxValue"], 

                gvRow.Cells[1].Text, 

                gvRow.Cells[2].Text);

        }

        obj.Rows.Add(

           this.TextBox1.Text,

             this.TextBox2.Text, );

        this.gv.DataSource = obj;

        this.gv.DataBind();

        

    }

and getemptyTable is

 private static DataTable GetEmptyTable()

    {

        var obj = new DataTable();

     

        obj.Columns.Add("MinValue");

        obj.Columns.Add("MaxValue");

    

        return obj;

    }

Please Help