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.io.*;
 public class Solution {
		static FastScanner scr=new FastScanner();
//		static Scanner scr=new Scanner(System.in);
		static PrintStream out=new PrintStream(System.out);
		static StringBuilder sb=new StringBuilder();
		
		
		
		
		static class pair{
			int x;int y;
			pair(int x,int y){ 
				this.x=x;this.y=y;
				
			}
		}
		static class triplet{
			long x;
			int y;
			int z;
			triplet(long x,int y,int z){
				this.x=x;
				this.y=y;
				this.z=z;
			}
		}static class DSU{
			int []a=new int[26];
			int []rank=new int[26];
			 void start() {
				for(int i=0;i<26;i++) {
					a[i]=i;
				}
			}
			  int find(int x) {
				 
				 if(a[x]==x) {
					 return x;
				 }
				 return find(a[x]);
			 }
			  void union(int x,int y) {
				  int xx=find(x);
				  int yy=find(y);
				  a[xx]=yy;
			  }
		}
		static class FastScanner {
			 BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
			StringTokenizer st=new StringTokenizer("");
		 	String next() {
		 		while (!st.hasMoreTokens())
		 			try {
		 				st=new StringTokenizer(br.readLine());
		 			} catch (IOException e) {
		 				e.printStackTrace();
		 			}
		 			return st.nextToken();
			} long gcd(long a,long b){
				if(b==0) {
					return a;
				}
					
				return gcd(b,a%b);
			}
		 	int[]  sort(int a[]) {
				 
		        int multiplier = 1, len = a.length, max = Integer.MIN_VALUE;
		        int b[] = new int[len];
		        int bucket[];
		        for (int i = 0; i < len; i++) if (max < a[i]) max = a[i];
		        while (max / multiplier > 0) {
		            bucket = new int[10];
		            for (int i = 0; i < len; i++) bucket[(a[i] / multiplier) % 10]++;
		            for (int i = 1; i < 10; i++) bucket[i] += (bucket[i - 1]);
		            for (int i = len - 1; i >= 0; i--) b[--bucket[(a[i] / multiplier) % 10]] = a[i];
		            for (int i = 0; i < len; i++) a[i] = b[i];
		            multiplier *= 10;
		        }
		        return a;
		    }
		 	 long modPow(long base,long exp) {
				if(exp==0) {
					return 1;
				}
				if(exp%2==0) {
					long res=(modPow(base,exp/2));
					return (res*res);
				}
				return (base*modPow(base,exp-1));
			}
			int []reverse(int a[]){
				int b[]=new int[a.length];
				int index=0;
				for(int i=a.length-1;i>=0;i--) {
					b[index]=a[i];
				}
				return b;
			}
			int nextInt() {
				return Integer.parseInt(next());
			}
			int[] readArray(int n) {
				int[] a=new int[n];
				for (int i=0; i<n; i++) a[i]=nextInt();
				return a;
			}
			long[] readLongArray(int n) {
				long  [] a=new long  [n];
				for (int i=0; i<n; i++) a[i]=nextLong();
				return a;
			}
			long nextLong() {
				return Long.parseLong(next());
			}
		}
		
		static ArrayList<pair>a;
		static int prime[];
		static void solve() {
			int n=scr.nextInt();
			
			int []a=scr.readArray(n);
			for(int i=0;i<n-1;i++) {
				if(Math.abs(a[i]-a[i+1])>=2) {
					out.println("NO");
					return;
				}
			}
			out.println("YES");
		}
		static int  MAX = Integer.MAX_VALUE;
		static int  MIN = Integer.MIN_VALUE;
		public static void main(String []args) {
//			int t=scr.nextInt();
//			while(t-->0) {
//				solve();
//			}
			solve();
			out.println(sb);
		}
	}
