模拟
import java.util.*;
public class Main{
static int a[]=new int[1010];
public static void main(String[] args){
Scanner sc=new Scanner(System.in);
int n=sc.nextInt();
int flag=1;
for(int i=1;i<=n;i++) a[i]=sc.nextInt();
for(int i=1;i<n;i++){//等于n超出界线
if(a[i+1]!=a[i]+1) flag=0;
}
if(flag==1) System.out.print("YES");
else System.out.print("NO");
}
}