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Score : $525$ points

问题陈述

一个正整数序列 $X = (X_1, X_2, \ldots)$（可能为空）被称为1122序列，当且仅当它满足以下三个条件：（1122序列的定义与问题D中相同。）

• $\lvert X \rvert$ 是偶数。这里，$\lvert X \rvert$ 表示 $X$ 的长度。
• 对于每个满足 $1\leq i\leq \frac{|X|}{2}$ 的整数 $i$，$X_{2i-1}$ 和 $X_{2i}$ 相等。
• 每个正整数在 $X$ 中要么不出现，要么恰好出现两次。也就是说，$X$ 中包含的每个正整数在 $X$ 中恰好出现两次。

给定一个由正整数组成的长度为 $N$ 的序列 $A = (A_1, A_2, \ldots, A_N)$，请打印出 $A$ 的一个**子序列（不一定是连续的）**的最大长度，该子序列是一个1122序列。

以上为大语言模型 kimi 翻译，仅供参考。

Problem Statement

A sequence $X = (X_1, X_2, \ldots)$ of positive integers (possibly empty) is called a 1122 sequence if and only if it satisfies all of the following three conditions: (The definition of a 1122 sequence is the same as in Problem D.)

• $\lvert X \rvert$ is even. Here, $\lvert X \rvert$ denotes the length of $X$.
• For each integer $i$ satisfying $1\leq i\leq \frac{|X|}{2}$, $X_{2i-1}$ and $X_{2i}$ are equal.
• Each positive integer appears in $X$ either not at all or exactly twice. That is, every positive integer contained in $X$ appears exactly twice in $X$.

Given a sequence $A = (A_1, A_2, \ldots, A_N)$ of length $N$ consisting of positive integers, print the maximum length of a subsequence (not necessarily contiguous) of $A$ that is a 1122 sequence.

Constraints

• $1\leq N \leq 2 \times 10^5$
• $1\leq A_i \leq 20$
• All input values are integers.

Input

The input is given from Standard Input in the following format:

$N$

$A_1$ $A_2$ $\ldots$ $A_N$

Output

Print the maximum length of a (not necessarily contiguous) subsequence of $A$ that is a 1122 sequence.

Sample Input 1

7
1 3 3 1 2 2 1

Sample Output 1

4

For example, choosing the $1$-st, $4$-th, $5$-th, and $6$-th elements of $A$, we get $(1, 1, 2, 2)$, which is a 1122 sequence of length $4$.
There is no longer subsequence that satisfies the conditions for a 1122 sequence, so the answer is $4$.

Sample Input 2

1
20

Sample Output 2

0