# -*- coding: utf-8 -*-

import sys
import copy

sys.setrecursionlimit(1000000)


# input = sys.stdin.readline

# ~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~

def get_divisors(x):
    ret = []
    i = 1
    while i * i <= x:
        if x % i == 0:
            ret.append(i)
            if x // i != i:
                ret.append(x // i)
        i += 1
    ret = sorted(ret)
    return ret


A, B = map(int, input().split())

divisors = get_divisors(A + B)

ans = 100000000000000000000

for C in divisors:
    if (A + C) % B == 0 and (B + C) % A == 0 and A != B and B != C and A != C:
        ans = min(ans, C)


print(ans if ans != 100000000000000000000 else -1)