adjacent

import java.util.Scanner;
 
     	public class adjacent {
     		public static void main(String[] args){
 
     			Scanner input = new Scanner(System.in);
     			int positions = input.nextInt();
     			int myArray[] = new int [positions];
 
     			for (int i=0; i < positions;i++){
     			myArray[i] = input.nextInt();
     			}
 
     			int theanswer = adjacent(myArray);
     			if (theanswer == 1){
     			System.out.println("true");	
     			}
     			else if (theanswer == 0){
     				System.out.println("false");
     			}
 
 
     		}
	     		public static int adjacent(int[] myArray){
	     			 Scanner input = new Scanner (System.in);
                          int x = 0;
                          int x1 = 0;
                          int x2 = 0;
                          int x3 = 0;
	     			 for (int j=0; j <= myArray.length - 3; j++){
	     				x1 = myArray[j];
	     				x2 = myArray[j+1];
	     				x3 = myArray[j+2];
                              
                              }

                              int fox = x3 - x2;
                              int fox1 = x2 - x3;
	     				if (fox == 1) 	{
	     					x = 1; 
	     					}
						else if(fox == -1){
							x = 1; 
                              }
                              else {
                                   x = 0;
                              }


	     					
	
                              return x;
	     			}
 
	     		}