n=int(input())
a=*map(int,input().split()),
l=0
r=n
while l+1<n and a[l]<a[l+1]:
l+=1
while r>0 and a[r-2]>a[r-1]:
r-=1
print('YNEOS'[len({*a[l:r]})!=1::2])#include <bits/stdc++.h>
using namespace std;
int main()
{
int n;
cin >> n;
vector<int> a(n);
for (int i = 0; i < n; i++)
{
cin >> a[i];
}
int inc = 1, c = 0, d = 0;
for (int i = 1; i < n; i++)
{
if (a[i] > a[i - 1])
{
if (c == 1 || d == 1)
{
cout << "NO" << endl;
return 0;
}
}
else if (a[i] == a[i - 1])
{
if (d == 1)
{
cout << "NO" << endl;
return 0;
}
c = 1;
}
else
{
d = 1;
}
}
cout << "YES" << endl;
return 0;
}
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