geometry - Calculate midpoint from a series of latitude and longitude coordinates

I've got a series of longitude and latitude coordinates which represent a building outline

e.g.

-0.5485381346101759,53.2285150736142
-0.5482220594232723,53.22842450827133
-0.5482298619861881,53.22841205254449

...(intermediate points not listed)...

-0.5483123769301657,53.22882101914848

How can I work out the midpoint? I've found tutorials that show how to do it if you've got three coordinates (e.g. http://mathforum.org/library/drmath/view/68373.html), but in many cases I've got more than three.

Thank you