Calculating The Sum Of A Calculated Column

Hi,

I'm writing a query that calculates values for each id, then I need to sum up all the values for each id and put them in another field, here is what I wrote but SQL cannot understand the column that I calculeted

select id,term_cd,
case when RIGHT(term_cd, 1) IN ('6') and substring(term_cd, 2,4) <= 2004) then '1'
when substring(term_cd, 2,4) <= 2004 and RIGHT(term_cd, 1) = 5 then '0.5'
else '1' end as term_count,
SUM(term_count) AS ttd_enrolled
group by id,term_cd

SQL gives me an error "invalid column name tern_count" in the line where I calculate the sum.
what's wrong with the query? or should I calculate the term_count in an inner query?

Appretiate your help