Hi, coders. I welcome you to Like Shiyan Simulation Round 2. This round will be held on OJ Essential. As usual, you can take any 1h of your spare time.

Hints:
1. Problems: 4 (100-100-50-50)
2. Time: 1h
3. Difficulty: Around Div2D
4. 1 from Atcoder,2 from LOJ,1 from Topcoder(Modified)
5. OI Rule
6. Contest Writer:XGN

Editorial

Problem A - The Lucky Sum

If N>17, the answer is always "YES". How to get this result?

First you need to know that if an integer N can be represented as the lucky sum, N must satisfy the only condition: (a,b are integers).

Then because gcd(4,7)=1, so the maximum number that cannot be obtained is lcm(4,7)-4-7=17

Source/Extra Topcoder - THELUCKYSUM

Expected Difficulty d2a

Problem B - The Game

Here

Source Atcoder AGC007D

Expected Difficulty d2c for 95%, d2e for AC

Problem C - The Gloves

Here

Source BOI 2008

Expected Difficulty d2c