Conditional variance in bivariate normal

Let the random vector $(X_1, X_2)$ follow the bivariate normal distribution with $$E(X_1)=E(X_2)=1,\quad \mathrm{Var}(X_2)=4\mathrm{Var}(X_1)=4\quad\text{and}\quad \mathrm{Cov}(X_1, X_2)=1.$$ Then, $\mathrm{Var}\left(X_1+X_2 \mid X_1=\frac{1}{2}\right)$ equals __________