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.util.*;
import java.math.*;
import java.io.*;

public class CF952C {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		int[] a = new int[n];
		for(int i = 0 ; i < n ; i++)
			a[i] = scan.nextInt();
		boolean passed = true;
		for(int i = 0 ; i < n - 1 ; i++)
			if(Math.abs(a[i] - a[i + 1]) >= 2)
				passed = false;
		List<Integer> list = new ArrayList<>();
		for(int i = 0 ; i < n ; i++)
			 list.add(a[i]);
		for(int i = 0 ; i < n ; i++) {
			int max = 0;
			int id = -1;
			for(int j = 0 ; j < list.size() ; j++) {
				if(list.get(j) > max) {
					max = list.get(j);
					id = j;
				}
			}
			list.remove(id);
			for(int j = 0 ; j < list.size() - 1 ; j++) {
				if(Math.abs(list.get(j) - list.get(j + 1)) >= 2)
					passed = false;
			}
		}
		Arrays.sort(a);
		for(int i = 0 ; i < n - 1 ; i++)
			if(Math.abs(a[i] - a[i + 1]) >= 2)
				passed = false;
		if(passed)
			System.out.println("YES");
		else
			System.out.println("NO");
	}
}
