# frozen_string_literal: true

# rubocop:todo Metrics/PerceivedComplexity
# rubocop:todo Metrics/AbcSize
def solve # rubocop:todo Metrics/CyclomaticComplexity
  return A.size < B.size ? B : A unless A.size == B.size

  A.chars.map(&:to_i).zip(B.chars.map(&:to_i)).each do |x, y|
    return A if x == 4 && y == 7 || !(x == 7 && y == 4) && (x > y)
    return B if x == 7 && y == 4 || x < y
  end
end
# rubocop:enable Metrics/AbcSize
# rubocop:enable Metrics/PerceivedComplexity

A, B = gets.chomp.split

puts solve