using alias in select statement

 Hi,

I have this query:

select systemtype, SUM(CASE WHEN isflash(narserno)=1 THEN 1 ELSE 0 END)*100/COUNT(*) flash,

100-SUM(CASE WHEN isflash(narserno)=1 THEN 1 ELSE 0 END)*100/COUNT(*)

from systemconfig GROUP BY SYSTEMTYPE;

however, i want to replace it with:

select systemtype, SUM(CASE WHEN isflash(narserno)=1 THEN 1 ELSE 0 END)*100/COUNT(*) flash,

100-flash

from systemconfig GROUP BY SYSTEMTYPE;

it does not work and gives me this error:

ORA-00904: "FLASH": invalid identifier

00904. 00000 -  "%s: invalid identifier"

*Cause:    

*Action:

Error at Line: 27 Column: 4

my question is: why does it fail and how to make it work with out running that whole complex sum statement again.