Mr. Kitayuta, the Treasure Hunter

题意： 在一个有n个点的岛屿上，有n个点有宝藏。 现在给你一个d。 开始你在0 第一步，你将跳到d处 之后你可以选择跳c-1,c,c+1步(c为你上一步跳的步数) 每跳到某处可获得该地宝藏 询问最大宝藏数 感谢@Harryheqg 提供的翻译

The Shuseki Islands are an archipelago of $ 30001 $ small islands in the Yutampo Sea. The islands are evenly spaced along a line, numbered from $ 0 $ to $ 30000 $ from the west to the east. These islands are known to contain many treasures. There are $ n $ gems in the Shuseki Islands in total, and the $ i $ -th gem is located on island $ p_{i} $ . Mr. Kitayuta has just arrived at island $ 0 $ . With his great jumping ability, he will repeatedly perform jumps between islands to the east according to the following process: - First, he will jump from island $ 0 $ to island $ d $ . - After that, he will continue jumping according to the following rule. Let $ l $ be the length of the previous jump, that is, if his previous jump was from island $ prev $ to island $ cur $ , let $ l=cur-prev $ . He will perform a jump of length $ l-1 $ , $ l $ or $ l+1 $ to the east. That is, he will jump to island $ (cur+l-1) $ , $ (cur+l) $ or $ (cur+l+1) $ (if they exist). The length of a jump must be positive, that is, he cannot perform a jump of length $ 0 $ when $ l=1 $ . If there is no valid destination, he will stop jumping. Mr. Kitayuta will collect the gems on the islands visited during the process. Find the maximum number of gems that he can collect.

The first line of the input contains two space-separated integers $ n $ and $ d $ ( $ 1<=n,d<=30000 $ ), denoting the number of the gems in the Shuseki Islands and the length of the Mr. Kitayuta's first jump, respectively. The next $ n $ lines describe the location of the gems. The $ i $ -th of them ( $ 1<=i<=n $ ) contains a integer $ p_{i} $ ( $ d<=p_{1}<=p_{2}<=...<=p_{n}<=30000 $ ), denoting the number of the island that contains the $ i $ -th gem.

Print the maximum number of gems that Mr. Kitayuta can collect.

4 10
10
21
27
27

3

8 8
9
19
28
36
45
55
66
78

6

13 7
8
8
9
16
17
17
18
21
23
24
24
26
30

4

In the first sample, the optimal route is 0 $ → $ 10 (+1 gem) $ → $ 19 $ → $ 27 (+2 gems) $ →...&#x1; $ In the second sample, the optimal route is 0 $ → $ 8 $ → $ 15 $ → $ 21 $ → $ 28 (+1 gem) $ → $ 36 (+1 gem) $ → $ 45 (+1 gem) $ → $ 55 (+1 gem) $ → $ 66 (+1 gem) $ → $ 78 (+1 gem) $ →... $ In the third sample, the optimal route is 0 $ → $ 7 $ → $ 13 $ → $ 18 (+1 gem) $ → $ 24 (+2 gems) $ → $ 30 (+1 gem) $ →... $