Go back to the exact post page number

I use pagination in my index.php and I have many posts per page. Every post have a button to go back to home. The problem is that when I visit a post in a page 5 for example, the button always send me on the first page.

I’m using this: <a href="<?php echo home_url(); ?>">back</a>

Is there a simple way to track the post page position, and use it to go back properly?