I swear this is probably the most simple thing ever but I have been trying to find the answer but dont seem to be able to get a solution. I have a questionnaire and each question I have created a score integer variable for the result. I am then wanting to sum the variables to give me the total score. I have the following flow:

But I get the following error........

I am sure it is something simple but any help would be much appreciated!
Many thanks in advance.
Alex