How to delete bill number auto Increment in windows APP. C#

Question

How to delete bill number auto Increment

 

Suppose I Have 50 Bill Total and I delete bill number 10 to 25. Now when creating new bill number

 

Bill Number 10 and then complete 25 bills

 

After create new bill 51 number

 

int Billno = billno("Select ISNULL(Max(Billno+1),1) From BillForm

 

private int billno(string p)

{

Int32 Billno = 0;

//s = Billno;

cmd = new SqlCommand(p, con);

int k;

con.Open();

if (txtbill.Text != "")

{

Billno = Convert.ToInt32(txtbill.Text);

k = Billno;

s = k;

k = (Int32)cmd.ExecuteScalar();

con.Close();

return Billno;

}

else

{

Billno = s;

Billno = (Int32)cmd.ExecuteScalar();

con.Close();

return Billno;

}

}