require 'prime'
require 'set'

def _divisors(primes, k)
  Enumerator.new do |y|
    if primes.size == k
      y << 1
    else
      p, e = primes[k]
      _divisors(primes, k + 1).each do |d|
        (e + 1).times { |e1| y << p ** e1 * d }
      end
    end
  end
end

def divisors(n)
  _divisors(Prime.prime_division(n), 0).to_a
end

a, b = gets.split(" ").map(&:to_i)
ab = a + b
list = divisors(ab)

c = -1
list.each do |i|
  next if i == a || i == b
  ac, bc = a + i, b + i
  if ac % b == 0 && bc % a == 0
    c = i
    break
  end
end
puts c