problem in my coding

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Gridview

i want a column for checkbox in the gridview and next column for sequence using textbox when i checked the checkbox then sequence textbox show 1 next time if i checked another checkbox in the list then show in the sequence textbox value is 2 and so on.........my coding is that but problem arises for 11 value it show 10 when times of 12 it show 10 this process is for 11 to 19 when times of 20 then it show correct

checkbox sequence commodity
fill 1 a
blank blank b
fill 2 c
blank blank d
blank blank e
blank blank f
fill 3 g

protected void grd_comm_grp_SelectedIndexChanged(object sender, EventArgs e)
{
     CheckBox cbSelection = (CheckBox)sender;
    GridViewRow row = (GridViewRow)cbSelection.NamingContainer;
    TextBox CenterSequence = (TextBox)row.FindControl("CenterSequence");
    // If Checked
    if (cbSelection.Checked)
    {
        // Enable the TextBox
        CenterSequence.Enabled = true;

        // Find Rows with sequence value
      var  ValidSequences = grd_comm_grp.Rows.Cast<GridViewRow>().Where(a => ((TextBox)a.FindControl("CenterSequence")).Text != "").ToList();
        // If there are any sequences, increment by 1
        if (ValidSequences.Count > 0)


            CenterSequence.Text = (Convert.ToInt32(ValidSequences.Max(a => ((TextBox)a.FindControl("CenterSequence")).Text)) + 1).ToString();



        // If new, put 1
        else
            CenterSequence.Text = "1";

    }