Consider the following sulphure based oxoacids. $$
\mathrm{H}_2 \mathrm{SO}_3, \mathrm{H}_2 \mathrm{SO}_4, \mathrm{H}_2 \mathrm{~S}_2 \mathrm{O}_8 \text { and } \mathrm{H}_2 \mathrm{~S}_2 \mathrm{O}_7 \text {. } $$
Amongst these oxoacids, the number of those with peroxo ( $\mathrm{O}-\mathrm{O}$ ) bond is $\_\_\_\_$ .