If statement for admin page

I would use get_current_screen(); to do this. It returns a WP_Screen object.

You should remember that this function will return NULL if it’s called before admin_init action.

In your case it should look like this:

$screen = get_current_screen();

if ( $screen && $screen->action == 'add'
    && $screen->base == 'post' && $screen->post_type == 'slideshow' ) {
    // do your stuff
}