I have a polygon:

Polygon[{{0, 200 }, {200, 100}, {500, 300}, {100, 700}}] 

How can I figure out its area? The docs page does not have any example.

So far I've reached this point with no success:

Needs["Polytopes`"] Area[Polygon[{{0, 200 }, {200, 100}, {500, 300}, {100, 700}}]]; 

Concerning Area, the documentation states:

Area[polygon]
$\ \ \ \ \ $ gives the area of polygon polygon, when the edges of polygon have unit length.

So it is possible that my question really should be how to set unit length on edges of polygon?