N, M = [int(s) for s in input().split()]
ans = 0

for i in range(31):
    match (i % 2, (N >> i) & 1, (M >> i) & 1):
        case (0, 0, 0):
            ans |= 0 << i
        case (0, 0, 1):
            print(-1)
            exit()
        case (0, 1, 0):
            ans |= 0 << i
        case (0, 1, 1):
            ans |= 1 << i
        case (1, 0, 0):
            ans |= 0 << i
        case (1, 0, 1):
            ans |= 1 << i
        case (1, 1, 0):
            print(-1)
            exit()
        case (1, 1, 1):
            ans |= 0 << i

print(ans)