nginx - SSL certificate for www. and non-www domain

What SSL certificate should I buy to get my domain work?
I use nginx web server and want that all users requests go to https.

http://example.com -> https://example.com
http://www.example.com -> https://example.com
https://example.com -> https://example.com

https://www.example.com -> https://example.com

Do I have to buy 2 certificates for both domains example.com and www.example.com?
Or may I buy only one certificate with subdomain support?
What does SAN mean in SSL-certificates and how can it help me?

What nginx config do I need to get desire redirection?

Thanks.