from(select round(sum(bytes)/1024/1024) mbytes_free,tablespace_name

 from dba_free_space

 group by tablespace_name) a,

(select round(sum(bytes)/1024/1024) mbytes_alloc,tablespace_name

 from dba_data_files

 group by tablespace_name) b

 where a.tablespace_name(+)=b.tablespace_name

and b.tablespace_name='USERS'

 /