import sys

def input(): return sys.stdin.readline().strip()
def list2d(a, b, c): return [[c] * b for i in range(a)]
def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]
def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]
def ceil(x, y=1): return int(-(-x // y))
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]
def Yes(): print('Yes')
def No(): print('No')
def YES(): print('YES')
def NO(): print('NO')
sys.setrecursionlimit(10 ** 9)
INF = 10 ** 18
MOD = 10 ** 9 + 7
EPS = 10 ** -10

def divisors(N: int) -> set:
    """ 約数の列挙・個数 """
    from math import sqrt

    s = {1, N}
    for i in range(2, int(sqrt(N))+1):
        if N % i == 0:
            s.add(i)
            s.add(N // i)
    return s

a, b = MAP()

divs = sorted(divisors(a+b))
for c in divs:
    if (a + c) % b == 0 and (b + c) % a == 0:
        print(c)
        exit()
print(-1)