C. Almost Arithmetical Progressiontime limit per test1 secondmemory limit per test256 megabytesinputstandard inputoutputstandard outputGena loves sequences of numbers. Recently, he has discovered a new type of sequences which he called an almost arithmetical progression. A sequence is an almost arithmetical progression, if its elements can be represented as:  a1 = p, where p is some integer;  ai = ai - 1 + ( - 1)i + 1·q (i > 1), where q is some integer. Right now Gena has a piece of paper with sequence b, consisting of n integers. Help Gena, find there the longest subsequence of integers that is an almost arithmetical progression.Sequence s1,  s2,  ...,  sk is a subsequence of sequence b1,  b2,  ...,  bn, if there is such increasing sequence of indexes i1, i2, ..., ik (1  ≤  i1  <  i2  < ...   <  ik  ≤  n), that bij  =  sj. In other words, sequence s can be obtained from b by crossing out some elements.InputThe first line contains integer n (1 ≤ n ≤ 4000). The next line contains n integers b1, b2, ..., bn (1 ≤ bi ≤ 106).OutputPrint a single integer — the length of the required longest subsequence.ExamplesInput23 5Output2Input410 20 10 30Output3NoteIn the first test the sequence actually is the suitable subsequence. In the second test the following subsequence fits: 10, 20, 10.
Input23 5
Output2
['brute force', 'dp', '*1500']