When we use "using" block in our code; do we need to close the connection explicitly?

Hi all, please See my Code snippet:

My Question is: When I use  "using()"  do i explicitly need to close the connection object?  

As i read in documentation, objects will be cleared automatically. Am I doing something wrong?

protected void cmdRegister_Click(object sender, EventArgs e)
   {
       int Userid = 0;
       using (SqlConnection Cn = new SqlConnection(str))
       {

           using (SqlCommand Cmd = new SqlCommand("Insert_User"))
           {
               Cmd.CommandType = CommandType.StoredProcedure;
               Cmd.Parameters.AddWithValue("@Username", txtUsername.Text);
               Cmd.Parameters.AddWithValue("@Password", txtPassword.Text);
               Cn.Open();
               Userid = Convert.ToInt32(Cmd.ExecuteScalar());
               Cn.Close();
           }
       }
   }