The "2" is actually a parameter of the rule, and is the desired inflation target. OP is just hardcoding it in because the Fed's stated inflation target is 2%. If you leave it parameterized you can't simplify the equation further, as the 0.5 distributes over the desired inflation target parameter as well.

For that matter, the 0.5 is also a parameter, and is basically saying "Weight the goals of full employment and stable prices equally." If, say, you wanted to weight Fed policy 80% toward controlling inflation (to a target of 2%) and 20% toward maximizing employment, the equation would be r = p + 0.2y + 0.8(p - 2) + 2.