Merge two strings using user-defined function

DELIMITER $$DROP FUNCTION IF EXISTS `test`.`dateformat1`$$
CREATE FUNCTION dateformat1 ( monthyear  nvarchar(80), day nvarchar(80), n Int)
	RETURNS  nvarchar(80)BEGIN
 Declare s1 varchar(80);
	Declare s2 varchar(80);
	declare i int;
	set i=1;
	set s1 = '';
	loop1: loop 
	              
			SET s1 = s1 + substring(monthyear,i,1);
                        IF i = n   then
			      LEAVE loop1;
			ELSE
				set i = i+1;
				ITERATE loop1;
                        END IF;
        END loop loop1;



        SET s1 = s1 + day + substring(monthyear,i,length(monthyear)-i+1 );
	RETURN s1;

END$$DELIMITER ;
select dateformat1 ('09/2009','01/',4);