The reaction of sulphur in alkaline medium is the below:
$ \mathrm{S}_8(\mathrm{~s})+\mathrm{a} \mathrm{OH}^{-}(\mathrm{aq}) \rightarrow \mathrm{b} \mathrm{~S}^{2-}(\mathrm{aq})+\mathrm{c} \mathrm{~S}_2 \mathrm{O}_3^{2-}(\mathrm{aq})+\mathrm{d} \mathrm{H}_2 \mathrm{O}(\ell) $
The values of ' $a$ ' is $\_\_\_\_$ (Integer answer)