.htaccess - If modified since with (replace www) modrewrite-rule -

i use domain http://mydomain.com , not http://www.mydomain.com,
use rule

rewritecond %{http_host} ^www\.(.*)$ rewriterule ^(.*)$ http://%1/$1 [l,r=301] 

my problem is, 301 redirect -> when visit domain without www

any ideas fix this?

my goal is:

http://www.mydomain.com -> 301 http://mydomain.com -> 304 not modified 

thanks in advance!