source_code: import java.util.Scanner;
   
  public class pillar {
  	public static void main(String[] args) {
  		Scanner sc=new Scanner(System.in);
  		int n=sc.nextInt();
  		int a[]=new int[200005];
  		for (int i=1;i<=n;i++) 
  		a[i]=sc.nextInt();
  		for (int i=2;i<n;i++) 
  		if (a[i-1]>a[i]&&a[i]<a[i+1]) {
  			System.out.println("NO");
  			return;
  		}
  		System.out.println("YES");
  	}
  }
  
  