20-24

$${S_f = k \ln (N!) - 2k \ln [(N/2)!] \\ = k [N (\ln N) - N] - 2k [(N/2) \ln (N/2) - (N/2)] \\ = k [N (\ln N) - N - N \ln (N/2) + N] \\ = k [N (\ln N) - N (\ln N - \ln 2)] = N k \ln 2}$$