TheMemberFormerlyKnownAsAWOL:
Because that's the type that Serial.read returns?
Whilst it is true that the function returns an int, I bet that the OP does not know that, and in any case the sketch does not make use of the fact that the value could be negative.
Have a look at the examples in Serial Input Basics - simple reliable non-blocking ways to receive data. There is also a parse example to illustrate how to extract numbers from the received text.