import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(10 ** 7)


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, readline().split())
c = make_divisors(a + b)
for check in c:
    if check == a or check == b:
        continue
    if (check + a) % b == 0 and (check + b) % a == 0:
        print(check)
        exit()
print(-1)