Count Once - Checkboxes

Hi there - can someone help me with an issue please - 

I have a checklist in a slide and a "Next Slide" button that only becomes available when a certain number of documents have been read / boxes have been checked.  I have created a counter to count the number of ticks in the box and when the counter reaches 6, the state on the Next button changes from Disabled to Normal and the learner can move to the next page.  Could someone advise how to do this?  Currently I have my Counter variable set up as such for each check box : 

But I would like the counter to decrement when the checkbox is deselected and the status of the checkbox is "Normal" as opposed to "Selected".  I know I need a condition in here - I just don't know what condition I need.  It keeps counting even when I click in the box to deselect.  HELP PLEASE its driving me up the wall.  Thank you