For starters, you need to convert the strings to numbers. For example:

parseInt("${q://QID122_1//SelectedAnswer/Recode/1}")

Hi there, you might also be able to make use of Formula Variable Creation in the Data & Analysis tab or Math Operations in the Survey Flow.
https://www.qualtrics.com/support/survey-platform/data-and-analysis-module/data/add-new-fields/formula-variable-creation/
https://www.qualtrics.com/support/survey-platform/survey-module/editing-questions/piped-text/math-operations/

Thanks to both of you!!!!
@TomG: I already thried "parseINT" but it did not work. I thinkt that I might not use the right way to adress the values of a matrix question?
${q://QID122/SelectedAnswerRecode/1}
as piped text works completely fine but if I even use just one variable for my index I just get -1; so the matrix question is ignored
brandindex=parseInt(("${q://QID122//SelectedAnswerRecode/1}")-1)
If you see my mistake please let me know :-)

Tom_1842 : I tried your options too Thank you! But my problem is that I need to calulate on basis of the newly built variables as well (my formula is a little more complex so I need several steps). That's why I would prefer using Javascirpt .

Ah okay, JavaScript may be the way to go here, but Math Operations can also use newly built variables in calculations. If you create each variable in a separate Embedded Data element, the subsequent elements can reference any of the variables defined in the preceding elements.

TiBa ,
Your parens are incorrect. It should be:
brandindex=parseInt("${q://QID122//SelectedAnswerRecode/1}")-1
Also, just in case, in order to pipe the values, the script must be on a page after the matrix question.

TomG : True, thanks! with the right parens it worked Now I have all formulas in my javascript :-)
Tom_1842 Thank you too! I was not aware how much you can actually do with the embedded data! But as I need some conditions in my formulas in this case javascript code is "better to read". But it is still another work which I might use more often :-)