CF831A.Unimodal Array

Array of integers is unimodal, if:

• it is strictly increasing in the beginning;
• after that it is constant;
• after that it is strictly decreasing.

The first block (increasing) and the last block (decreasing) may be absent. It is allowed that both of this blocks are absent.

For example, the following three arrays are unimodal: [5, 7, 11, 11, 2, 1], [4, 4, 2], [7], but the following three are not unimodal: [5, 5, 6, 6, 1], [1, 2, 1, 2], [4, 5, 5, 6].

Write a program that checks if an array is unimodal.

整数数组被称为单峰的（unimodal），当且仅当满足以下条件：

• 它在开头严格递增；
• 接着为常数段（即所有元素相等）；
• 最后严格递减。

首段（递增段）和末段（递减段）可以不存在；允许这两段同时不存在。

例如，以下三个数组是单峰的：

5,\,7,\,11,\,11,\,2,\,1$$， $$4,\,4,\,2$$， $$7$$， 但以下三个数组**不是**单峰的： $$5,\,5,\,6,\,6,\,1$$， $$1,\,2,\,1,\,2$$， $$4,\,5,\,5,\,6$$。 请编写一个程序，判断给定数组是否为单峰数组。

The first line contains integer n (1 ≤ n ≤ 100) — the number of elements in the array.

The second line contains n integers _a_1, _a_2, ..., a__n (1 ≤ a__i ≤ 1 000) — the elements of the array.

第一行包含一个整数 $n$（$1 \leq n \leq 100$）—— 数组中元素的个数。

第二行包含 $n$ 个整数 $a_1,\,a_2,\,...,\,a_n$（$1 \leq a_i \leq 1\,000$）—— 数组的元素。

Print "YES" if the given array is unimodal. Otherwise, print "NO".

You can output each letter in any case (upper or lower).

如果给定数组是单峰的，则输出 "YES"；否则输出 "NO"。

你可以以任意大小写（大写或小写）输出每个字母。

In the first example the array is unimodal, because it is strictly increasing in the beginning (from position 1 to position 2, inclusively), that it is constant (from position 2 to position 4, inclusively) and then it is strictly decreasing (from position 4 to position 6, inclusively).

在第一个例子中，该数组是单峰的，因为它在开头严格递增（从位置 1 到位置 2，包含两端），然后保持恒定（从位置 2 到位置 4，包含两端），最后严格递减（从位置 4 到位置 6，包含两端）。