Displaying values at each end of slider

I just can't think through the trigger logic to do this:

At the left end of a slider I have a text box displaying the value of a slider's variable. At the right end I want a text box to display the 'opposite' value (for example, if the slider is moved to '3' in a 1-10 scale, the opposite end should show '7' - and so on).

I have created a new variable for the right hand end, and I think the math would be that this should always have the value 10 minus the slider variable. I just can't program a trigger to do it!