CF 424 - A

// LightOJ always needs this format for sure..so I made a copy of it...
#include <bits/stdc++.h>
#include<vector>
#define ll                                      long long int
#define fi                                      freopen("in.txt", "r", stdin)
#define fo                                      freopen("out.txt", "w", stdout)
#define m0(a) memset(a , 0 , sizeof(a))
#define m1(a) memset(a , -1 , sizeof(a))
#define inf LLONG_MAX
#define min3(a,b,c) min(a,min(b,c))
#define max3(a,b,c) max(a,max(b,c))
#define ones(mask)  __builtin_popcount(mask) /// __builtin_popcount : it's a built in function of GCC. Finds out the numbers of 1 in binary representation.
#define mx 150000
//#define m 1000000007

using namespace std;

char c[1010] = {} ;

int main()
{
    ll n ;
    scanf("%lld",&n) ;

    if(n==1)
    {
        printf("YES\n") ;
        return 0 ;
    }

    for(ll i = 0 ; i < n ; i++)
    {
        scanf("%lld",&c[i]) ;
    }

    ll x = 0 ;

    /// 1 = big , 2 = eq , 3 = small ;

    for(ll i = 1 ; i < n ; i++)
    {
        if(x == 0)
        {
            if(c[i] > c[i-1])
            {
                x = 1 ;
            }
            else if(c[i] == c[i-1])
            {
                x = 2 ;
            }
            else if(c[i] < c[i-1])
            {
                x = 3 ;
            }
        }
        else if(x == 1)
        {
            if(c[i] > c[i-1])
            {
                x = 1 ; /// not needed
            }
            else if(c[i] == c[i-1])
            {
                x = 2 ;
            }
            else
            {
                x = 3 ;
            }
        }
        else if(x == 2)
        {
            if(c[i] > c[i-1])
            {
                printf("NO\n") ;
                return 0 ;
            }
            else if(c[i] == c[i-1])
            {
                x = 2 ;
            }
            else
            {
                x = 3 ;
            }
        }
        else if(x == 3)
        {
            if(c[i] > c[i-1])
            {
                printf("NO\n") ;
                return 0 ;
            }
            else if(c[i] == c[i-1])
            {
                printf("NO\n") ;
                return 0 ;
            }
            else
            {
                x = 3 ;
            }
        }
    }

    printf("YES\n") ;

    return 0 ;
}