CF2137C Maximum Even Sum

You are given two integers $$$a$$$ and $$$b$$$. You are to perform the following procedure: First, you choose an integer $$$k$$$ such that $$$b$$$ is divisible by $$$k$$$. Then, you simultaneously multiply $$$a$$$ by $$$k$$$ and divide $$$b$$$ by $$$k$$$. Find the greatest possible even value of $$$a+b$$$. If it is impossible to make $$$a+b$$$ even, output $$$-1$$$ instead.

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 first line of each test case contains two integers $$$a$$$ and $$$b$$$ ($$$1 \\leq a,b \\leq a\\cdot b \\leq 10^{18})$$$.

For each test case, output the maximum even value of $$$a+b$$$ on a new line.

In the first test case, it can be shown it is impossible for $$$a+b$$$ to be even. In the second test case, the optimal $$$k$$$ is $$$2$$$. The sum is $$$2+4=6$$$.