Good day, Codeforces!

Given a polygon with N vertices, where the vertices(x[i], y[i]) are listed in counterclockwise order, you need to check if an angle is concave in the polygon. To better understand what is meant, refer to the provided illustrations:

By (ABC) I will mean the angle where B is the vertex of the angle, and A and C are the sides of the angle.

In that pic, (AFE) and (EDC) are concave angles.

Sorry for my bad English, any help will be appreciated.

Hi, guys!

Can solving problems only from IOI-style olympiads (like POI, JOI, IOI, IZHO, etc.), help me to be ready for any IOI-style olympiad? And which OI would you suggest to solve?

Personally, I've heard that POI is great for getting better.

Please, share with your opinion, 'cause I want to hard-work in summer.