ORA-00060: Deadlock

User2	User1
	update  usef_wait set a=25 where a=18 ;
update  usef_wait set a=35 where a=19 ;
	update  usef_wait set a=36 where a=19 ;
update  usef_wait set a=26 where a=18 ;

ORA-00060: deadlock detected while waiting for resource

update  usef_wait set a=36 where a=19 ; è ROLLBACK