How to redirect an address to (return 301 https ) - except for *.subdomains?

Question

Nginx

Can you please tell me how to redirect the site to https , but only for the main domain example.com , without auto-subdomains like *.example.com ?
Now Nginx has this setting:
listen 123.12.12.123:80;
return 301 https://$host$request_uri;