The area of the quadrilateral formed by these four points isn’t 2, is it?
pts = {{0, 0}, {1, -1}, {1, 1}, {4, 0}}
Area[Polygon[pts]]
The points are in the wrong order
pts = {{0, 0}, {1, -1}, {4, 0}, {1, 1}};
Area @ Polygon[pts]
4
To illustrate Frank’s point:
Thank you! So it’s determined by the order of coordinates. Thanks!