What url causes wordpress to use archive.php?

I have a blog with two post types. One is default ‘post’ and the other one is custom post type ‘video’. In permalink options I choosen ‘custom structure’:

/posts/%postname%/

I created ‘archive.php’ in the theme directory. I supposed that url ‘/posts/’ should cause wordpress to use ‘archive.php’ as template for posts loop page. But I’m getting ‘Page not found’ title with ‘index.php’ as template file. How to fix it?

By the way, url ‘/videos/’ chosen for videos by setting option

'rewrite' => array('slug' => 'videos', 'with_front' => false)
in the ‘register_post_type()’ function uses ‘archive-video.php’ as assumed.