EIf you think about it, combining -2 and 4 makes 2, and combining -6 and 8 makes two as well. That means that there's a value of two more for each two terms. Since this goes up in an interval of 2, and there's a total of 24 at the end, there's a total of 12 numbers and 6 of them are positive. Since it goes up 2 for every positive number, that means the total is 12. Sorry if I was confusing, I can clarify if you need it.
