The deadlock diner

ERROR:  deadlock detected
DETAIL: Process 812 waits for ShareLock on transaction 9911; blocked by process 640.
        Process 640 waits for ShareLock on transaction 9907; blocked by process 812.
CONTEXT: while updating tuple (3,17) in relation "accounts"

BEGIN;
UPDATE accounts SET balance = balance - $1 WHERE id = $2;
UPDATE orders   SET status = 'settled' WHERE account_id = $2 AND status = 'paid';
COMMIT;

BEGIN;
UPDATE orders   SET status = 'paid' WHERE id = $1;
UPDATE accounts SET balance = balance + $2 WHERE id = $3;
COMMIT;