CF2085C Serval and The Formula

You are given two positive integers $ x $ and $ y $ ( $ 1\le x, y\le 10^9 $ ). Find a non-negative integer $ k\le 10^{18} $ , such that $ (x+k) + (y+k) = (x+k)\oplus (y+k) $ holds $ ^{\text{∗}} $ , or determine that such an integer does not exist. $ ^{\text{∗}} $ $ \oplus $ denotes the [bitwise XOR operation](https://en.wikipedia.org/wiki/Bitwise_operation#XOR).

Each test contains multiple test cases. The first line contains the number of test cases $ t $ ( $ 1 \le t \le 10^4 $ ). The description of the test cases follows. The only line of each test case contains two integers $ x $ and $ y $ ( $ 1\le x, y\le 10^9 $ ) — the given integers.

For each test case, output a single integer $ k $ ( $ 0\le k\le 10^{18} $ ) — the integer you found. Print $ -1 $ if it is impossible to find such an integer. If there are multiple answers, you may print any of them.

In the first test case, since $ (2 + 0) + (5 + 0) = (2 + 0) \oplus (5 + 0) = 7 $ , $ k=0 $ is a possible answer. Note that $ k=4 $ is also a possible answer because $ (2 + 4) + (5 + 4) = (2 + 4) \oplus (5 + 4) = 15 $ . In the second test case, $ (x+k)\oplus (y+k) = (6+k)\oplus (6+k) = 0 $ . However, $ (x+k)+(y+k) > 0 $ holds for every $ k \ge 0 $ , implying that such an integer $ k $ does not exist.