test2.java

/* package whatever; // don't place package name! */

import java.io.*;

public class myCode
{
  public static void main (String[] args) throws java.lang.Exception
  {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String input = br.readLine();
    System.out.println(input);
    
    int n = Integer.valueOf(input);
     int [] arr = new int [n];
      String [] strArr = br.readLine().split("\\s+"); 
      for(int i = 0; i < n; i ++) {
        arr[i] = Integer.valueOf(strArr[i]);
      }   
      smooth(arr);        
      
  }
  
  public static void smooth(int [] arr) {
    int n = arr.length;
    boolean isSmooth = true;
    for(int i = 0; i < n - 1; i++){
      if(Math.abs(arr[i] - arr[i+1]) > 1){
         isSmooth = false;
         break;
      }
    }
    if(isSmooth) {
      System.out.println("YES");
    }else {
      System.out.println("NO");
    }
  }
  
}