Just can't get this right.

I have -2+4/3-8/9+16/27... and want to write the sum by useing sigma.

I guess n is infinity, but after that it stopes for me. Only thing i can see is that i can multiply -2*(-2/3) to get 4/3 and so on. I tryed google to find somewhere i could learn how to think to make the formula but could not find much that help me.

Anyone can help me?