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

if A.gcd(B) >= 2
  puts -1
else
  l = A * B
  checked = Array.new(l + 1, false)

  0.step(l, A) do |a|
    checked[a] = true

    a.step(l, B) do |b|
      checked[b] = true
    end
  end

  0.step(l, B) do |b|
    checked[b] = true

    b.step(l, A) do |a|
      checked[a] = true
    end
  end

  puts checked.count(false)
end