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.Arrays;
import java.util.Collections;
import java.util.StringTokenizer;

public class Test {
	

    
	public static void main(String[] args) {
		MyScanner sc = new MyScanner();
		int n = sc.nextInt();  
		int[] a = new int[n];
 		for (int i = 0; i < n; i++) {
 			a[i] = sc.nextInt();
 		}
 		boolean fl = true;
 		for (int i = 1; i < n; i++) {
 			if (Math.abs(a[i]-a[i-1])>=2) {
 				fl = false;
 			}
 		}
		System.out.println(fl?"YES":"NO");
	}
	
	 public static class MyScanner {
	      BufferedReader br;
	      StringTokenizer st;
	 
	      public MyScanner() {
	         br = new BufferedReader(new InputStreamReader(System.in));
	      }
	 
	      String next() {
	          while (st == null || !st.hasMoreElements()) {
	              try {
	                  st = new StringTokenizer(br.readLine());
	              } catch (IOException e) {
	                  e.printStackTrace();
	              }
	          }
	          return st.nextToken();
	      }
	 
	      int nextInt() {
	          return Integer.parseInt(next());
	      }
	 
	      long nextLong() {
	          return Long.parseLong(next());
	      }
	 
	      double nextDouble() {
	          return Double.parseDouble(next());
	      }
	 
	      String nextLine(){
	          String str = "";
		  try {
		     str = br.readLine();
		  } catch (IOException e) {
		     e.printStackTrace();
		  }
		  return str;
	      }

	   }
}
