Redox Reactions

What is the change in oxidation number of selenium in the following redox reaction?

\(\mathrm{SeO}_{3(\text { (a) })}^{2-}+\mathrm{Cl}_{2(\mathrm{~g})}+2 \mathrm{OH}^{-} \longrightarrow \mathrm{SeO}_{4(\text { (a) })}^{2-}+2 \mathrm{Cl}_{(\text {(a) })}^{-}+\mathrm{H}_2 \mathrm{O}\)