How to make a redirect with an exception for a folder?

Question

Nginx

To remove index.php from url in nginx after server added to the first line:

# block access to /index.(php|htm|html) 
if ($request_uri ~ "/index.(php|html?)") { 
      rewrite ^ /$1 permanent; 
}

How to make an exception for the /bitrix/admin/ folder
?
Or maybe there is another way to remove index.php for the main page?