Uncorrelated linear combinations

Let $(X,Y)$ be a random vector with $\operatorname{Var}(X)=\operatorname{Var}(Y)=1$ and $\operatorname{Cov}(X,Y)=-0.5$. For which one of the following values of $b$, the random variables $U=bX+Y$ and $V=X+bY$ are uncorrelated?