import math._

object Main {
  def calc(a: String, b: String): String = {
    if (a.length < b.length) return b
    if (a.length > b.length) return a

    for (i <- 0 to a.length-1) {
      if ((a(i) == '4' || a(i) == '7') &&
          (b(i) == '4' || b(i) == '7')) {
        if (a(i) == max(a(i), b(i)))
          return b
        else
          return a
      }

      if (a(i) < b(i)) return b
      if (a(i) > b(i)) return a
    }
    return a
  }

  def main(args: Array[String]) {
    val sc = new java.util.Scanner(System.in)
    val a, b = sc.next

    println(calc(a, b))
  }
}