Is it posible to solve it using only the first 2 coordinates? I
already found a solution but it uses 3 coordinates and I have to read
all coordinates first.

For each line segment (x1; y1) (x2; y2) I calculate cross product of
2 vectors:
(0; 0) (x1; y1)   and (0; 0) (x2; y2)

Summing all cross products gives double square of polygon but with
one exception: it is positive if vertices are situated ccw and
negative otherwise (cw).

  Just look for the upper, the lower and the most to the right
points. Then label then acording on how you found them (e.g.: if you
find the lower first then you give it a lower number than the upper
point, I used "i" for this).

  Then you just have to check the sequence. If you have any problem
understanding this, I can post my solution.

PS: This algorithm has a small bug that you don't have to fix in
order to get AC!

Vallery777@yahoo.com

   My program uses the following method:
   I choose the upper, lower and rightmost points and then sort them
in the order they were visited. Suppose their coordinates are
(x1,y1), (x2,y2), (x3,y3), where 1 was the first visited, 2 - second
and 3 - third.
   Then I put them in a martix
   x1 y1 1
   x2 y2 1
   x3 y3 1
and calculate D=x1*y2*1-x3*y2*1. If D>0, the 3 points are oriented
clockwise. When D<0, then orientation is counterclockwise.
   But what happens when D=0? And why is my method wrong for the
sample input?
   Pls help me!
   Vallery777@yahoo.com