Crystal reports - summarization of fields

Question

Hi...I am trying to summarize fields and one is a score..I want to add scores if the topics are matching(topic is another column).It is like...

Topic  Score  Summarized column
A         10
A         20        30
B         40        40
C         60     
C         70        130

I want to display the third column.Is it preferable to use a formula or summarised fields.I tried it with summarised dialog box but it is only giving me count and not
the sum .It works for max/minimum value however.

Formula may be more appropriate but (this is another question)when ever I use numerical variables as the result in any formula written it returns an error that"Astring is required here" or "A number or currency value needed"...I am unable to proceed..
Kindly respond.