N, M = map(int, input().split())

ans = 0
for i in range(30):
    n = N>>i & 1
    m = M>>i & 1
    if i%2 == 0:
        if n == 0 and m == 1:
            exit(print(-1))
        if m == 1:
            ans |= 1<<i
    else:
        if n == 1 and m == 0:
            exit(print(-1))
        if n == 0 and m == 1:
            ans |= 1<<i

print(ans)