a, b = gets.split.map(&:to_i) s = a+b ans = [] (1..s).each do |i| break if i*i > s next if s % i > 0 [i, s/i].each do |c| if c != a && c != b && (b+c) % a == 0 && (c+a) % b == 0 ans.push(c) end end end if ans.size > 0 puts ans.min else puts -1 end