time limit per test1 second
memory limit per test256 megabytes
inputstandard input
outputstandard output
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.

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

The second line contains n integers a1, a2, …, an (1 ≤ ai ≤ 1 000) — the elements of the array.

Output
Print “YES” if the given array is unimodal. Otherwise, print “NO”.

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

Examples
input
6
1 5 5 5 4 2
output
YES
input
5
10 20 30 20 10
output
YES
input
4
1 2 1 2
output
NO
input
7
3 3 3 3 3 3 3
output
YES
Note
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).

題目大意：有一個序列，如果滿足左部嚴格上升（可以為空），中間恆定，右部嚴格下降（可以為空），那麼就輸出YES，否則輸出NO。
解題思路：按照規則走一遍，如果沒走完，那麼就NO，否則YES。

#include<iostream>
#include<cstdio>
using namespace std;
const int MAXN=105;
const int INF=0x3f3f3f3f;
int a[MAXN];

int main()
{
    int n;
    while(cin>>n)
    {
        for(int i=1;i<=n;i++)
            cin>>a[i];
        a[n+1]=INF;
        int p=2;
        while(a[p]>a[p-1]) p++;
        while(a[p]==a[p-1]) p++;
        while(a[p]<a[p-1]) p++;
        if(p<=n) cout<<"NO"<<endl;
        else cout<<"YES"<<endl;
    }
    return 0;
}