import sys
from typing import List, Tuple

MOD = 998244353

int1 = lambda x: int(x) - 1
input = lambda: sys.stdin.readline().rstrip('\n')
ii = lambda: int(input())
vi = lambda: list(map(int, input().split()))
vi1 = lambda: list(map(int1, input().split()))


def dbg(*args, **kwargs):
    print(*(repr(arg) for arg in args), *(f'{k}: {repr(v)}' for k, v in kwargs.items()),
          sep='; ', file=sys.stderr, flush=True)


def main():
    n, m = map(int, input().split())

    ans = 0
    for i in range(30):
        nb, mb = (n >> i) & 1, (m >> i) & 1
        if i % 2 == 0:
            if mb == 0:
                continue
            if nb == 1:
                ans += 1 << i
            else:
                return -1
        else:
            if nb == mb: continue
            if nb == 0:
                ans += 1 << i
            else:
                return -1
    return ans


def _start():
    if (ret := main()) is not None:
        print(*ret) if isinstance(ret, List) or isinstance(ret, Tuple) else print(ret)


if __name__ == '__main__':
    _start()