4
90 91 90 91
 4
54 54 54 55
 10
55 39 93 42 97 40 36 38 11 97
 3
90 39 98
 5
21 57 40 94 17
 8
46 4 30 85 52 6 84 13
 6
47 100 96 2 96 43
 4
61 28 3 81
 5
14 91 91 91 84
 3
22 23 24
 1
13
 9
13 80 13 38 98 85 11 73 74
 10
95 51 52 8 44 39 77 17 96 88
 3
40 40 40
 4
16 17 17 18
 3
68 67 67
 4
83 100 81 75
 9
71 71 70 70 71 70 69 70 71
 10
9 9 9 10 10 9 8 8 9 9
 6
87 86 86 86 85 86
 5
64 63 63 62 61
 3
76 9 12
 4
61 60 60 60
 7
23 37 39 8 72 31 85
 10
67 67 67 67 67 67 67 67 67 67
 4
92 18 29 93
 4
14 15 15 16
 10
79 84 22 38 23 22 33 42 13 96
 7
20 21 95 73 49 98 53
 10
28 61 23 73 61 33 45 55 18 43
 9
15 14 14 13 13 12 13 13 14
 5
46 46 47 46 45
 7
35 91 87 78 17 71 63
 3
69 26 73
 10
16 17 16 15 14 15 16 17 16 15
 9
95 7 97 61 90 7 30 65 39
 10
2 83 18 65 58 95 37 51 86 47
 6
59 59 60 60 59 58

YES YES NO NO NO NO NO NO NO YES YES NO NO YES YES YES NO YES YES YES YES NO YES NO YES NO YES NO NO NO YES YES NO NO YES NO NO YES
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        ArrayList<Integer> list1=new ArrayList<Integer>();
        ArrayList<Integer> list2=new ArrayList<Integer>();
        for (int i=0; i<n; i++) list1.add(sc.nextInt());
        boolean b=true;
        for (int i=0; i<n-1; i++){
            if (Math.abs(list1.get(i)-list1.get(i+1))>1) b=false;
        }
        if (b) {
            int i0 = 0;
            int max = 0;
            for (int i = 0; i < n; i++) {
                i0 = 0;
                max = 0;
                for (int j = 0; j < list1.size(); j++) {
                    if (list1.get(j) > max) {
                        max = list1.get(j);
                        i0 = j;
                    }
                }
                list2.add(list1.get(i0));
                list1.remove(i0);
            }
            b=true;
            for (int i=0; i<n-1; i++){
                if (Math.abs(list2.get(i)-list2.get(i+1))>1) b=false;
            }
            if (b) System.out.println("YES"); else System.out.println("NO");
        } else System.out.println("NO");
        sc.close();
    }
}
