def make_divisors(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 = make_divisors(A+B)

for C in div:
    if A != C and B != C and (A+C) % B == 0 and (B+C) % A == 0:
        print(C)
        break
else:
    print(-1)