def comp(a, b)
  if a.length != b.length
    return a.length <=> b.length
  end
  (0...a.length).each do |i|
    if a[i] == b[i]
      next
    elsif a[i] == 4 && b[i] == 7
      return 1
    elsif a[i] == 7 && b[i] == 4
      return -1
    else
      return a[i] <=> b[i]
    end
  end
  return 0
end
a, b = gets.chomp.split(" ").map{|s|s.chars.map &:to_i}
if comp(a, b) == -1
  puts b*''
else
  puts a*''
end