Centroid of a irregular polygon???????

Known the vertices of an IRREGULAR polygon (x1,y1), (x2,y2), (x3,y3)...... and so on how to find its centroid?

I tried to divide the polygon into differrent triangles with a common vertex [say (x1,y1)] and finding their centroids,but i cudn't go further... :o ... what's the next step?........Please help me out.

Centroid of a polygon with n sides

what i meant was how to find the centroid of an irregular polygon with n sides..........i shud have mentioned the vertices as (x1,y1),(x2,y2),(x3,y3),.................(xn.yn). sorry for that.............