As far as I know there is no dynamic way to do arithmetic calculations (variant of dynamic SELECT ... WHERE (cond_syntax) ). You can only dynamicaly create report (INSERT REPORT).
Also there is this "Internally released" function EVAL_FORMULA which works with arithmetic expresions in string format. Program to evaluate formula in a string - ABAP Development - SCN Wiki
But I would rather go simple step-by-step way:
CASE operator.
WHEN '+'.
result = a + b.
....
ENDCASE.