Add/subtract from variable depending on state selected/normal

Hi all,

I'm building an activity where users can click on different parts of a paragraph, e.g. to identify the topic sentence. Users click the various sentences to highlight them and click again to deselect (SELECTED/NORMAL states). What I need is a variable to act as a counter that increases by 1 when an incorrect sentence is selected, and subtracts 1 again if they deselect it. (After they click a "submit" button the layer they see will be determined by the number of incorrect sentences they included, hence the need for a counter.)

I've tried adding "Adjust variable" triggers to add/subtract 1 when any of the rectangles over the incorrect sentences are selected - screenshots below - but it only ever seem to do one or the other, only add or only subtract.

Any suggestions on what I'm doing wrong... or better ways of doing this? Thanks in advance!