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.io.IOException;
import java.util.*;

public class Main2 {

    public static void main(String[] args) throws IOException {
        Scanner in = new Scanner(System.in);

        List<Integer> raviolli = new ArrayList<>();
        int n = in.nextInt();
        for (int i = 0; i < n; i++) {
            raviolli.add(in.nextInt());
        }

        while (!raviolli.isEmpty()) {
            int max = raviolli.get(0);
            for (int i = 1; i < raviolli.size(); i++) {
                int current = raviolli.get(i);
                if (current > max) {
                    max = current;
                }
            }

            for (int i = 0; i < raviolli.size(); i++) {
                int item = raviolli.get(i);
                if (item == max) {
                    if (i > 0 && (item - raviolli.get(i - 1)) > 1) {
                        System.out.println("NO");
                        in.close();
                        return;
                    }

                    if (i < raviolli.size() - 1 && (item - raviolli.get(i + 1)) > 1) {
                        System.out.println("NO");
                        in.close();
                        return;
                    }
                }
            }

            int finalMax = max;
            raviolli.removeIf(i -> i == finalMax);
        }

        System.out.println("YES");
    }
}
