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