def d(n):
  divisors = []
  for i in range(1, int(n**0.5)+1):
    if n % i == 0:
      divisors.append(i)
      if i != n // i:
        divisors.append(n//i)

  divisors.sort()
  return divisors

a, b = map(int, input().split())
div = d(a+b)

for c in div:
  if a != c and b != c and (a+c) % b == 0 and (b+c) % a == 0:
    print(c)
    exit()

print(-1)