My solution is geometrical, it passes all of my tests, except of one, where one side has a length of 0.
But I got WA on test #2, so:
 1) Can a polygon have a side with length of 0?
 2) Can somebody give me any tests for this problem?

1) I'm pretty sure it can't, since otherwise my AC solution has a big chance to divide by zero.
2) 3
0 0
3 0
0 3
answer: 1.9308
Yeah, this is a pretty simple test but it actually helped me to overcome that WA #2.

@Chitanda Eru, but 1.9308 aint correct.

UPD: it is.

Edited by author 30.09.2013 02:21

but N — the number of polygon vertices (4 ≤ N ≤ 50)