object Yuki293 extends App{
  def compare(a :String, b:String) :String = {
    if(a.length > b.length) return a
    else if (a.length < b.length) return b

    def compareImpl(x :String, y:String, i:Int) :String = {
      if(i >= x.length) return x
      
      val xi = x.charAt(i).asDigit
      val yi = y.charAt(i).asDigit

      if(xi == 4 && yi == 7) x
      else if(xi == 7 && yi == 4) y
      else if(xi == yi) compareImpl(x, y, i + 1)
      else if(xi >= yi) x
      else y 
    }

    compareImpl(a, b, 0)
  }

  val input = new java.util.Scanner(System.in)
  val a = input.next
  val b = input.next
  val result = compare(a, b)
  println(result)
}