Michael Phelps had collected a total of 23 golds at the Olympics

If Michael Phelps swims $8$ meters upstream at $\SI[per-mode=symbol]{1}{\meter\per\second}$ relative to the shore and back downstream to the same point at $\SI[per-mode=symbol]{5}{\meter\per\second}$ relative to the shore, what is his average speed (in $\si[per-mode=symbol]{\meter\per\second}$ )?