For a dimerization reaction
$$ 2 \mathrm{~A}(\mathrm{~g}) \rightarrow \mathrm{A}_2(\mathrm{~g}) $$ at $298 \mathrm{~K}, \Delta \mathrm{U}^{\circ}=-20 \mathrm{~kJ} \mathrm{~mol}^{-1}, \Delta \mathrm{~S}^{\circ}=-30$ $\mathrm{JK}^{-1} \mathrm{~mol}^{-1}$, then the $\Delta \mathrm{G}^{\circ}$ will be $\_\_\_\_$ J.