Need formula to show amount of change (+ or -)

How do I create a formula to show me the amount of change between two tests my students have taken? For example, if they scored a 99 on test 1 and a 97 on test 2, I'd like Excel to display the change as a -2 because the score changed 2 points. If they got a 100 on test 2 I'd like Excel to display the change as +1. Thanks!