# Differences between HMAC and HKDF in a specific case

I want to use a key derivation function to derive a key to another one.

I think HKDF is the easiest solution in my case.

The following picture gives a clear definition of it : http://webee.technion.ac.il/~hugo/kdf/hkdf-fig.jpg

In the case where the number of key bits request from the KDF is equal to the hash output length (i.e. L = k in the definition) do I have to compute 2 HMAC :

PRK = HMAC(XTS, SKM) and K(1) = HMAC(PRK, CTXinfo)

or can I directly compute :

K(1) = HMAC(SKM, CTXinfo) ?