def sw(n)
  n.to_s.chars.map{|c|
    case c
    when "4" then "7"
    when "7" then "4"
    else c
    end
  }.join.to_i
end

def main(a, b)
  p sw(a) < sw(b) ? b : a
end


if __FILE__ == $0
  main(*gets.split(" ").map(&:to_i))
  # main(4, 7)
end