javascript - How do I direct mobile users to a mobile site only once? -

i direct users coming site on mobile device mobile website. simple enough, add script header of home-page:

<script type="text/javascript">     if (screen.width <= 699) {     document.location = "mobile-site-url";     } </script> 

however, on mobile site, want give users option of going full site. how do that? script above redirects them every time try access page on mobile device.

how redirect them first time, not when click on "visit full site" link on mobile site?

thanks in advance!