Okay, the reason the code included the unary plus was to more directly represent the resulting expression, but I'm guessing the compiler isn't going to respect that intent even if it could somehow understand it, so it will probably remove the symbols and just parse it all the same. Unlike with the unary plus, the unary minus can be "fixed" by wrapping it in parentheses. The end result is of course the exact same, but the intent is perhaps a bit clearer this way. We want to try to coerce the compiler into creating an expression with "add a negative value", just for the sake of "correctness" at the runtime evaluation level, but even if that isn't what will actually happen, the explicit code is still a bit easier to read. While unary plus is easy to disregard, "fixing" an unnecessary unary minus would mean having to change the binary operator before it, which muddies the intent of the expression.
MobArena 
MobArena is an arena-style minigame for Spigot-based Minecraft servers
Getting Started
If you don't want to compile the plugin yourself, grab a release artifact from one of these two sites:
The wiki here on Github should have all the information you need to get started using the plugin.
Getting Help
If you run into problems or need help with something, feel free to hop on the MobArena Discord server to have a chat: Instant Invite
Remember, though, that this communication channel is idle chat, which means you likely won't get a response immediately. It could take anything from a couple of seconds to a day, but your message will be seen.
Contributing
Found a bug, got a suggestion, or want to dig into the code base? There are many ways to contribute to the project!
Get started here: Contributing