window.onscroll = function() {
      let scrolled = window.pageYOffset || document.documentElement.scrollTop;
      (scrolled >= 100) ? document.querySelector('.header').classList.add('head-scroll') :
                          document.querySelector('.header').classList.remove('head-scroll');
    }

var cities = {
      bra: ["Сан-Паулу", "Рио-де-Жанейро"],
      rus: ["Москва", "Санкт-Петербург"],
      ind: ["Мумбаи", "Дели"],
      chn: ["Шанхай", "Пекин"],
      zaf: ["Йоханнесбург", "Кейптаун"]

    };

    var country = document.getElementById("country");
    var city = document.querySelector("#city");

    window.onload = selectCountry(city, cities);
    country.onchange = function() {
      selectCountry(city, cities);
    }

    function selectCountry(city, ev) {
      city.innerHTML = "";
      var c = country.value || "bra",
        o;
      for (let i = 0; i < ev[c].length; i++) {
        o = new Option(ev[c][i], ev[c][i], false, false);
        city.add(o);
      };
    }