def inp() a=gets.chomp.split(" ").map(&:to_i)end 
def copy(a) Marshal.load(Marshal.dump(a)) end
def kaizyo(n)(n < 2)? 1 : (2..n).inject(:*) end
def scount(a,b) a.each{|n|b[n]+=1} end
a,b = gets.chomp.split(" ")
a = a.split("")
b = b.split("")
unless(a.size == b.size)
  if(a.size > b.size)
    puts a.join
  else
    puts b.join
  end
  exit
end
a.map!(&:to_i)
b.map!(&:to_i)
a.size.times do |d|
  if([4,7].include?(a[d]) and [4,7].include?(b[d]))
    if(a[d] == 4 and b[d] == 7)
      puts a.join
      exit
    elsif(a[d] == 7 and b[d] == 4)
      puts b.join
      exit
    end
  else
    if(a[d] > b[d])
      puts a.join
      exit
    elsif(a[d] < b[d])
      puts b.join
      exit
    end
  end
end