Number#1 KimSeolho Defender's Things

Anyone know this manwha about A class or S class Doors suddenly popping up and when you enter through that door and clear whatever objective is behind it, ur class will be an A class or S class depending on the class of that door? if I remember correctly the Chapter 1 was about the mc getting an A class door but hes brother stole it from him instead but apparently the mc will get a S class door in the future?