.htaccess redirect if file exists

Hi,

I'm hoping this is an easy one but I can't figure it out.

Here's my current .htaccess file

RewriteEngine on

# do not do anything for already existing files
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule .+ - [L]

# Handle redirection to routes
RewriteRule     ^public/(.*) app/routes/public/www/$1 [L]
RewriteRule     ^admin/(.*) app/routes/admin/www/$1 [L]

# Handle framework requests
RewriteRule ^(.*)$ framework/index.php$1  [L]

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On these lines,
RewriteRule     ^public/(.*) app/routes/public/www/$1 [L]
RewriteRule     ^admin/(.*) app/routes/admin/www/$1 [L]
How do I set these to only redirect if the requested file actually exists at the new location? Otherwise continue to process the rest of the .htaccess file?

Bascially, I don't want to redirect /admin/ to app/roues/admin/www/ but /admin/css/style.css should redirect to app/routes/admin/www/css/style.css ok as it's an actual file.