Given the vertices of a simple polygon (its edges do not cross) as integer points [x, y], listed in order around the boundary, return twice its area as an integer.
- The order may be clockwise or counter-clockwise, and it may start at any vertex; the answer is never negative
- The polygon can be concave
- Returning twice the area keeps every value an integer, so nothing needs rounding
- Fewer than three vertices, or all vertices on one line, give
0 - Coordinates are integers up to
107 in absolute value - Do not change the input