text: window.onscroll = function () {
    myFunction();
  };
  
  var header = document.getElementsByTagName("nav")[0];
  var sticky = header.offsetTop;
  
  function myFunction() {
    if (window.pageYOffset > sticky) {
      header.classList.add("fixed-top");
    } else {
      header.classList.remove("fixed-top");
    }
  }
  