CF602B.Approximating a Constant Range

When Xellos was doing a practice course in university, he once had to measure the intensity of an effect that slowly approached equilibrium. A good way to determine the equilibrium intensity would be choosing a sufficiently large number of consecutive data points that seems as constant as possible and taking their average. Of course, with the usual sizes of data, it's nothing challenging — but why not make a similar programming contest problem while we're at it?

You're given a sequence of n data points _a_1, ..., a__n. There aren't any big jumps between consecutive data points — for each 1 ≤ i < n, it's guaranteed that |a__i + 1 - a__i| ≤ 1.

A range [l, r] of data points is said to be almost constant if the difference between the largest and the smallest value in that range is at most 1. Formally, let M be the maximum and m the minimum value of a__i for l ≤ i ≤ r; the range [l, r] is almost constant if M - m ≤ 1.

Find the length of the longest almost constant range.

当谢洛斯在大学参加一门实践课程时，他曾经需要测量某种效应的强度，该效应会缓慢趋近于平衡态。确定平衡强度的一个好方法是：选取足够多的连续数据点，使得这些数据点看起来尽可能恒定，然后取它们的平均值。当然，对于通常规模的数据而言，这并不具有挑战性——但既然我们正在设计编程竞赛题目，何不将它改编成一道类似的题目呢？

给定一个包含 $n$ 个数据点的序列 $a_1, \dots, a_n$。相邻数据点之间不会出现大幅跳跃——对每个满足 $1 \le i < n$ 的下标 $i$，均保证 $|a_{i+1} - a_i| \le 1$。

若某段数据点区间 $[l, r]$ 中最大值与最小值之差至多为 $1$，则称该区间为“近似恒定”（almost constant）。形式化地，令 $M$ 表示 $a_i$ 在 $l \le i \le r$ 范围内的最大值，$m$ 表示其最小值；则区间 $[l, r]$ 是近似恒定的，当且仅当 $M - m \le 1$。

请找出最长的近似恒定区间的长度。

The first line of the input contains a single integer n (2 ≤ n ≤ 100 000) — the number of data points.

The second line contains n integers _a_1, _a_2, ..., a__n (1 ≤ a__i ≤ 100 000).

输入的第一行包含一个整数 $n$（$2 \leq n \leq 100\,000$）—— 数据点的个数。

第二行包含 $n$ 个整数 $a_1,\,a_2,\,...,\,a_n$（$1 \leq a_i \leq 100\,000$）。

Print a single number — the maximum length of an almost constant range of the given sequence.

输出一个整数——给定序列中“几乎恒定”区间的最大长度。

In the first sample, the longest almost constant range is [2, 5]; its length (the number of data points in it) is 4.

In the second sample, there are three almost constant ranges of length 4: [1, 4], [6, 9] and [7, 10]; the only almost constant range of the maximum length 5 is [6, 10].

在第一个样例中，最长的几乎恒定区间是 [2, 5]；其长度（即其中的数据点个数）为 4。

在第二个样例中，存在三个长度为 4 的几乎恒定区间：[1, 4]、[6, 9] 和 [7, 10]；唯一长度为最大值 5 的几乎恒定区间是 [6, 10]。