---

#include <iostream>
#include <algorithm>
using namespace std;
int main(){
  int n;
  cin >> n;
  int arr[100];
  int arr_sort[100];
  for (int i = 0; i < n; i++){
    cin >> arr[i];
  }
  for (int i = 0; i < n; i++){
    arr_sort[i] = arr[i];
  }
  sort(&arr_sort[0], &arr_sort[n]);
  int trutru = 1;
  for (int i = 0; i < n; i++){
    if (arr_sort[i] != arr[i]){
      trutru = 0;
    }
  }
  if (trutru == 0){
    cout << "NO";
  }else{
    cout << "YES";
  }
}