Display alternative title from custom field if it exists?

I’m pretty new to WP templates. How can I get it to output the contents of a custom field “alt-title” if it exists and just the regular title if it doesn’t?

Here is the sort of thing i’m looking for in sudo code.

[if alt-title] <h1>alt-title</h1> [else] <h1>title</h1> [/if]

Thanks.