object Main extends App {
	val sc = new java.util.Scanner(System.in)
	val pw = new java.io.PrintWriter(System.out)
	
	def isDistinct(a: Seq[Int]): Boolean = a match {
		case Seq()   => true
		case x +: xs => xs.forall(x != _) && isDistinct(xs)
	}
	
	pw.println(if (isDistinct(Seq.fill(sc.nextInt)(sc.nextInt))) "YES" else "NO")
	pw.flush()
}