1 /*
 2     Name:
 3     Copyright:
 4     Author:
 5     Date: 2018/5/6 19:34:23
 6     Description:
 7 */
 8 #include <iostream>
 9 #include <cstdio>
10 #include <cstring>
11 using namespace std;
12 int arr[1005];
13 int main()
14 {
15     int n;
16     while (cin>>n) {
17         for (int i=0; i<n; i++) {
18             cin>>arr[i];
19         }
20         if (n == 1) {//1
21             cout<<"YES"<<endl;
22             continue;
23         }
24         int flag = 1;
25         for (int i=0; i<n; i++) {
26             if (arr[i] != arr[i-1]) flag = 0;
27         }
28         if (flag == 1) {//全相等
29             cout<<"YES"<<endl;
30             continue;
31         }
32
33         bool destory = false;
34         if (arr[0] == arr[1]) {
35             int tmp = arr[0];
36             int i = 2;
37             while (arr[i] == arr[i-1]) i++;
38             //不相等
39             for (; i<n; i++) {
40                 if (arr[i] >= arr[i-1]) {
41                     destory = true;
42                     break;
43                 } else {
44                 }
45             }
46             //判断完毕
47             if (destory)  {
48                 cout<<"NO"<<endl;
49             } else{
50                 cout<<"YES"<<endl;
51             }
52         } else {
53             int i=1;
54             //递增
55             while (arr[i] > arr[i-1] && i<n) i++;
56             //相等
57             while (arr[i] == arr[i-1] && i<n) i++;
58             //递减
59             while (arr[i] < arr[i-1] && i<n) i++;
60             if (i == n) {
61                 cout<<"YES"<<endl;
62             } else {
63                 cout<<"NO"<<endl;
64             }
65         }
66     }
67     return 0;
68 }