Thanks for the hint. I will try it.

The reason why we cannot recompile is because it's a very old program
(9 years old) and some source files are missing :-(

>
> since "F_1" and "SUM" are both 3 chars long, you could use a hex
> editor on the binary of your program, although this is of course not
> ideal and potentially risky.
>
> Why can't you recompile the code?