#!/bin/python

from __future__ import print_function

import os
import sys

#
# Complete the canModify function below.
#
def canModify(a):
    b=sorted(a)
    if(b==a):
        ans="YES"
    else:
        ans="NO"
    result=ans
    return result

if __name__ == '__main__':
    fptr = open(os.environ['OUTPUT_PATH'], 'w')

    n = int(raw_input())

    a = map(int, raw_input().rstrip().split())

    result = canModify(a)

    fptr.write(result + '\n')

    fptr.close()