I am far from a mathematical genius. However, this discards the aforementioned issue and issue raised in your comment that it must conveniently end with -1. If the 'final' term to this sequence is +1 then 2Z=2-2+2-2+2...+1. Same goes for the second 'Z.' I can't garuntee this is correct and without passing secondary school it is outside my general knowledge. Correct me if I am wrong or have mistakenly supported your argument...
Cheers