I found this on the internet (Yes, I know it's Java but I think it's the same concept that can be applied with C# too):

"Integers are numbers that have no fractional part. In Java, integers are represented in a 32-bit space. Furthermore, they are represented in 2's complement binary form, which means that one bit of these 32 is a sign bit. So, there are 231-1 possible values. So, there is no integer greater than the number 231-1 in Java."

Link: doc Java (Sorry I found a link in Italian)

So according to this concept, when you try to multiply by -1, the result should be 2147483648, but this value cannot be represented because it exceeds the maximum allowed value, consequently it is ignored leaving the usual result.

Finally I found for C# Math.negateExact();

it is practically like Java:

Link: doc Microsoft

I hope I helped you.