What wrong on my statement with APPEND_VALUES hint 11gR2?

$ cat redo-new.sql column REDO_SIZE new_value REDO_SIZE
select value REDO_SIZE
from v$mystat, v$statname
where v$mystat.statistic# = v$statname.statistic#
and v$statname.name = 'redo size';

$ cat redo-diff.sql
select (value - &REDO_SIZE) REDO_SIZE
from v$mystat, v$statname
where v$mystat.statistic# = v$statname.statistic#
and v$statname.name = 'redo size';

SQL> archive log list;
Database log mode              Archive Mode
Automatic archival             Enabled
Archive destination            USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence     161
Next log sequence to archive   163
Current log sequence           163

SQL> select * from v$version;

BANNER
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Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production