SP how can I sum the amount of all leftchild in parentid

I have this data and table structure,I am creating MySQL stored procedure to sum all the amount in the leftchild of the parentid, I don't have enough knowledge in stored procedure.and I need some help please because I'm lost.

CREATE TABLE IF NOT EXISTS `mytree` (
  `parentid` int(11) NOT NULL,
  `memberid` int(11) NOT NULL,
  `position` char(1) NOT NULL,
  `amount` decimal(10,2) NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;


INSERT INTO `mytree` (`parentid`, `memberid`, `position`, `amount`) VALUES
    (8, 27, 'R', 0.00),
    (8, 28, 'L', 0.00),
    (24, 26, 'R', 0.00),
    (0, 1, '', 5500.00),
    (24, 25, 'L', 0.00),
    (21, 24, 'L', 500.00),
    (21, 23, 'R', 0.00),
    (18, 20, 'R', 1500.00),
    (18, 19, 'L', 0.00),
    (15, 18, 'R', 2000.00),
    (15, 17, 'L', 0.00),
    (13, 16, 'L', 0.00),
    (13, 15, 'R', 2500.00),
    (12, 14, 'R', 0.00),
    (12, 13, 'L', 3000.00),
    (10, 12, 'R', 3500.00),
    (10, 11, 'L', 0.00),
    (7, 10, 'R', 4000.00),
    (7, 9, 'L', 0.00),
    (5, 8, 'R', 500.00),
    (5, 7, 'L', 4500.00),
    (1, 6, 'R', 0.00),
    (1, 5, 'L', 5500.00),
    (20, 22, 'R', 0.00),
    (20, 21, 'L', 1000.00);

here is my stored proc

CREATE DEFINER=`root`@`localhost` PROCEDURE `count_left_right_amount`(IN `p_memid` INT, OUT `tot_left_amount` DECIMAL(10,2))
    LANGUAGE SQL
    NOT DETERMINISTIC
    CONTAINS SQL
    SQL SECURITY DEFINER
    COMMENT ''
BEGIN

DECLARE row_total decimal(10,2);
DECLARE total decimal(10,2);
DECLARE m_left int;
DECLARE run_left int;
DECLARE p_id int;

set total =  0;

select memberid into p_id from mytree where position = 'L' AND parentid = p_memid;




WHILE  p_id != 0 DO

 select amount into row_total from mytree where parentid = p_id 
  GROUP BY parentid;

 select memberid into m_left from mytree where parentid = p_id;
 set total = total + row_total;
 set p_id = m_left;


END WHILE;


END

Example if the parentid is 5 ,so the leftchild is 7 then go down to his subtree sum it all,so the total amount in the left child is '22500'? please correct me if I'm wrong.

please help me.

Thank you in advance.