import math

def find_min_c(a, b):
    s = a + b
    # Generate all divisors of s
    divisors = set()
    for i in range(1, int(math.isqrt(s)) + 1):
        if s % i == 0:
            divisors.add(i)
            divisors.add(s // i)
    sorted_divisors = sorted(divisors)
    # Check each divisor in order
    for c in sorted_divisors:
        if c == a or c == b:
            continue
        # Check if (a + c) is divisible by b and (b + c) is divisible by a
        if (a + c) % b == 0 and (b + c) % a == 0:
            return c
    return -1

a, b = map(int, input().split())
print(find_min_c(a, b))