# Permutation

## 题意翻译

## 题目描述 给你一个1到n的排列，你需要判断该排列内部是否存在一个3个元素的子序列(可以不连续)，使得这个子序列是等差序列。 ## 输入输出格式 ### 输入格式： 第一行一个整数$n(n\leq 3\times10^5)$，表示给出的序列的长度。 第二行有$n$个整数，表示这个排列。 ### 输出格式： 如果存在，输出YES，反之输出NO。答案的大小写无所谓。

## 题目描述

You are given a permutation of numbers from $1$ to $n$ . Determine whether there's a pair of integers $a,b$ $(1<=a,b<=n; a≠b)$ such that the element ![](https://cdn.luogu.com.cn/upload/vjudge_pic/CF452F/79a61ff17360227fbc1ddd07b61b3ec2cf482ac4.png) (note, that it is usual division, not integer one) is between $a$ and $b$ in this permutation.

## 输入输出格式

### 输入格式

First line consists of a single integer $n$ ( $1<=n<=300000$ ) — the size of permutation. Second line contains $n$ integers — the permutation itself.

### 输出格式

Print "YES", if such a pair exists, "NO" otherwise (in both cases without quotes, the answer is case insensitive).

## 输入输出样例

### 输入样例 #1

4
1 3 4 2


### 输出样例 #1

NO


### 输入样例 #2

5
1 5 2 4 3


### 输出样例 #2

YES


## 说明

In the second example $2$ is between $1$ and $3$ . Additionally $4$ is between $3$ and $5$ .