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.

Hello, guys.

I just practice JOI problems, but I ran into a little problem. It turns out there are several JOI: JOI spring camp, JOI open contest, JOI final round.

Can someone explain which one is for what purpose.

And preferably, can you recommend which one to choose if you are preparing for IOI-style contests and your rating is about >= 1600?

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.

Hi, guys!

I've found something interesting that is not about CP. So if you don't like such blogs, don't read it.