i want to reset auto increment value when i delete some rows from database table

I want to reset increment value when i delete value from database table . Pno is primary key and here is the code what i have tried so far

    private void txtincr()
    {
        int a;
        if (txtPno.Text == "")
        {
            if (con.State == 0)
            {
                con.Open();
            }
            string str = "select Pno from Patient_Reg";
            SQLiteDataAdapter da = new SQLiteDataAdapter(str, con);
            DataSet ds = new DataSet();
            da.Fill(ds);
            if (ds.Tables[0].Rows.Count != 0)
            {
                a = ds.Tables[0].Rows.Count;
                a = a + 1;
                txtPno.Text = Convert.ToString(a);
                txtpname.Focus();
            }
            else
            {
                txtPno.Text = "1";
                txtpname.Focus();
            }
            con.Close();
        }
    }