A, B = gets.rstrip.split.map(&:chars)

if A.size != B.size
  puts A.size > B.size ? A.join : B.join
else
  0.upto(A.size - 1) do | i |
    if (A[i]=='4' && B[i]=='7') || (A[i]=='7' && B[i]=='4')
      puts A[i] < B[i] ? A.join : B.join
      exit
    else
      if    A[i] < B[i]
        puts B.join
        exit
      elsif A[i] > B[i]
        puts A.join
        exit
      else #A[i] == B[i]
        next
      end
    end
  end
end