Scroll top with definite section.js

$(window).scroll(function(){

      var contentNav = $('section.hero-section').height()

      if($(window).scrollTop() > contentNav ){
          $('#content-nav').addClass('fixedNav')
          $('#who-we-are').addClass('content-nav-fixed')
      }
      else{
          $('#content-nav').removeClass('fixedNav')
          $('#who-we-are').removeClass('content-nav-fixed')
      }
  })

// or 
var contentNav = $('#content-nav').offset().top - 0
 $(window).scroll(function(){

      if($(window).scrollTop() >= contentNav ){
          $('#content-nav').addClass('fixedNav')
          $('#who-we-are').addClass('content-nav-fixed')
      }
      else{
          $('#content-nav').removeClass('fixedNav')
          $('#who-we-are').removeClass('content-nav-fixed')
      }
  })