HMMT 2012 U3

Suppose $x$ and $y$ are real numbers such that $-1<x<y<1$. Let $G$ be the sum of the geometric series whose first term is $x$ and whose ratio is $y$, and let $G^{\prime}$ be the sum of the geometric series whose first term is $y$ and ratio is $x$. If $G=G^{\prime}$, find $x+y$.