Counter - pick many

Hi

I have an activity where the user has to select a number of hazards on a picture. There are 10. The counter counts how many hazards they have selected. However if they deselect a hazard, the counter still keeps adding 1, where it should really subtract 1.

I've created a "selected" stage, and added a trigger to the slide to add 1 to the counter each time the state of the object changes to selected.

I've tried adding another trigger to subtract 1 when the state of the object changes to normal but it doesn't work.

Does anyone have any ideas?

Many thanks

Shelley