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

public class Ravioli {

	public static void main(String args[]) throws NumberFormatException, IOException {
		
		Scanner input = new Scanner(System.in);
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		
		for(int index=0; index < 1; index++) {
			int size = Integer.parseInt(br.readLine());
			String s1[] = br.readLine().split(" ");
			int array[] = new int[size];
			boolean flag=true;
			array[0] = Integer.parseInt(s1[0]);
			for(int index1=1; index1 < size; index1++) {
				array[index1] = Integer.parseInt(s1[index1]);
				if(Math.abs(array[index1] - array[index1-1]) >= 2) {
				    
					System.out.println("NO");
					flag=false;
					break;
				}
			}
			if(flag)
				System.out.println("YES");
		}
		
	}
	
}