class Integer
  def divisor_list
    require 'prime'

    return [] if self <= 0
    return [1] if self == 1

    prime_division.map.with_index { |(base, k), i|
      s = i.zero? ? 0 : 1
      (s..k).map { |n| base ** n }
    }.inject { |res, e| res + res.flat_map { |t| e.map { |v| t * v } } }.sort
  end
end

A, B = gets.split.map(&:to_i).sort

(A + B).divisor_list.each do |x|
  next if x > A + B
  next if x < B - A
  next if x == A
  next if x == B

  if (A + x) % B == 0 && (B + x) % A == 0
    puts x
    exit
  end
end

puts -1