If f is holder continuous with a > 1, then f is constant?

Hello

I'm trying to prove that if f satisfies the holder condition, i.e.

there exists a constan K such that for all x,y in R : |f(x) - f(y)| <= K(|x - y|^a)

and a > 1

then f is a constant function.

I assume that the right direction is proving that the derivative of f is constantly zero, but I don't see why f has to be differentiable in the first place...and even assuming that f is differentiable I don't see why the derivative must be zero..