combining statements with jquery, and using media queries for a back to top button

I am always confused with writing statements in jQuery. I'm trying to get this back-top button to only appear on larger screen widths and hide on mobile.

How do I combine statements or change up my code properly to get it to work?

$(document).ready(function(){
    // Back to Top
    $("#back-top").hide(); // hide #back-top first

    // fade in after 500px down
    $(function () {
        $(window).scroll(function () {
          if ($(this).scrollTop() > 500 && screen.width >= 641) {
            $('#back-top').fadeIn();
          } else {
            $('#back-top').fadeOut();
          }
        });

        // scroll body to 0px on click
        $('#back-top').click(function () {
          $('body,html').animate({
            scrollTop: 0 }, 800); // 0px from top, 800 duration
          return false;
        });
    });

    if (screen.width < 641) {
        $("#back-top").hide();
    }
});