object Yuki293 extends App{
  val input = new java.util.Scanner(System.in)
  val a = input.next
  val b = input.next

  def convert(s :String) :String = s.map{ c => 
   c match {
    case '7' => '4'
    case '4' => '7'
    case x   => x
   }
  }.mkString

  val ai = convert(a).toInt
  val bi = convert(b).toInt

  val result = if(ai >= bi) a else b

  println(result)
}