How to make the aggregate computation for pick values fields

How to make the aggregate computation for pick values fields.

I am trying to make the computation for the sum(+) of a few pick values, i will choose few Story sizes(mentioned in the below command) the addition should able to see in the other field.

Facing eeror on below computation

aggregate("Sub Story", sum("Story Size"=="Very Tiny [1]"?1:("Story Size"=="Tiny [2]"?2:("Story Size"=="Very Small [3]"?3:("Story Size"=="Small [5]"?5:("Story Size"=="Modest [8]"?8:("Story Size"=="Medium [13]"?13:("Story Size"=="Large [20]"?20:("Story Size"=="Very Large [40]"?40:("Story Size"=="Epic [100]"?100:0))))))))))