Oh I didn't quite give the whole story.
you can think of positive numbers as hot numbers and negaive numbers as cold numbers.
If you take away heat it gets colder so - + = -
If you add cold into the room it gets colder + - = -
If you add heat it gets hotter + + = +
If you take cold away it also gets hotter - - = +
These are only for when the signs are so close together that you can put a circle around them with nothing else in the way.
Here is some more on this topic if you are interested :)
http://web2.0calc.com/questions/it-just-doesn-t-make-sense-to-me