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.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.io.OutputStream;
import java.io.Writer;
import java.io.IOException;
import java.util.ArrayList;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author palayutm
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        ProblemCRavioliSort solver = new ProblemCRavioliSort();
        solver.solve(1, in, out);
        out.close();
    }

    static class ProblemCRavioliSort {
        boolean check(ArrayList<Integer> a) {
            for (int i = 1; i < a.size(); i++) {
                if (Math.abs(a.get(i) - a.get(i - 1)) >= 2) {
                    return false;
                }
            }
            return true;
        }

        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int n = in.nextInt();
            ArrayList<Integer> arr = new ArrayList<>();
            for (int i = 0; i < n; i++) {
                arr.add(in.nextInt());
            }
            for (int i = 0; i < n; i++) {
                if (!check(arr)) {
                    out.println("NO");
                    return;
                }
                int mx = 0, p = -1;
                for (int j = 0; j < arr.size(); j++) {
                    if (arr.get(j) > mx) {
                        mx = j;
                        p = j;
                    }
                }
                arr.remove(p);
            }
            out.println("YES");
        }

    }

    static class OutputWriter extends PrintWriter {
        public OutputWriter(OutputStream out) {
            super(out);
        }

        public OutputWriter(Writer out) {
            super(out);
        }

        public void close() {
            super.close();
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] inbuf = new byte[1024];
        private int lenbuf = 0;
        private int ptrbuf = 0;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        private int readByte() {
            if (lenbuf == -1) throw new UnknownError();
            if (ptrbuf >= lenbuf) {
                ptrbuf = 0;
                try {
                    lenbuf = stream.read(inbuf);
                } catch (IOException e) {
                    throw new UnknownError();
                }
                if (lenbuf <= 0) return -1;
            }
            return inbuf[ptrbuf++];
        }

        public int nextInt() {
            int num = 0, b;
            boolean minus = false;
            while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-')) ;
            if (b == '-') {
                minus = true;
                b = readByte();
            }

            while (true) {
                if (b >= '0' && b <= '9') {
                    num = num * 10 + (b - '0');
                } else {
                    return minus ? -num : num;
                }
                b = readByte();
            }
        }

    }
}

