<div class="dropdown">
  <div class="dropdown__select"></div>
  <ul class="dropdown__list">
    <li>
      <div class="dropdown__list-left">
        <div class="dropdown__list-title">Взрослые</div>
        <div class="dropdown__list-subtitle">старше 12 лет</div>
      </div>
      <div class="dropdown__amount">
        <div class="dropdown__amount-minus">–</div>
        <div class="dropdown__amount-number">0</div>
        <div class="dropdown__amount-plus">+</div>
      </div>
    </li>
    <li>
      <div class="dropdown__list-left">
        <div class="dropdown__list-title">Дети</div>
        <div class="dropdown__list-subtitle">2-12 лет</div>
      </div>
      <div class="dropdown__amount">
        <div class="dropdown__amount-minus">–</div>
        <div class="dropdown__amount-number">0</div>
        <div class="dropdown__amount-plus">+</div>
      </div>
    </li>
    <li>
      <div class="dropdown__list-left">
        <div class="dropdown__list-title">Младенцы</div>
        <div class="dropdown__list-subtitle">до 2 лет, без места</div>
      </div>
      <div class="dropdown__list-right">
        <div class="dropdown__amount">
          <div class="dropdown__amount-minus">–</div>
          <div class="dropdown__amount-number">0</div>
          <div class="dropdown__amount-plus">+</div>
        </div>
      </div>
    </li>
    <li>
      <div class="checkbox">
        <input id="business" type="checkbox">
        <label for="business">Перелёт бизнес классом</label>
      </div>
    </li>
  </ul>

</div>

*, *:before, *:after {
  box-sizing: border-box;
}

body {
  margin: 30px;
  font-family: 'Roboto', sans-serif;
  font-weight: 300;
}

.dropdown {
  position: relative;
  max-width: 300px;
}

.dropdown__select {
  border: 1px solid #1ec7c2;
  border-radius: 5px;
  padding: 15px 20px 15px 15px;
  color: #e9eaec;
  font-size: 13px;
  position: relative;
  cursor: pointer;
  user-select: none;
}

.dropdown__select:after {
  content: "";
  display: block;
  border: solid #496885;
  border-width: 0 2px 2px 0;
  padding: 3px;
  position: absolute;
  right: 15px;
  top: 17px;
  transform: rotate(-135deg);
  transition: all 0.5s;
}

.dropdown__select.active:after {
  transform: rotate(45deg);
  top: 15px;
}

.dropdown__list {
  display: none;
  background-color: #142d4c;
  position: absolute;
  top: 100%;
  left: 0;
  width: 100%;
  height: auto;
  max-height: 300px;
  margin-top: 5px;
  border-radius: 5px;
  overflow-y: auto;
  padding: 20px;
}

.dropdown__list.active {
  display: block;
}

.dropdown__list > li {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 10px 5px;
  color: #fff;
}

.dropdown__list-title {
  font-size: 14px;
  color: #d3d1df;
}

.dropdown__list-subtitle {
  font-size: 12px;
  color: #3b5674;
  padding: 5px 0;
}

.dropdown__amount {
  display: flex;
  justify-content: space-between;
}

.dropdown__amount-minus,
.dropdown__amount-plus {
  border: 1px solid #1ec7c2;
  color: #1ec7c2;
  border-radius: 50%;
  width: 25px;
  height: 25px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 18px;
  line-height: 0.5;
  transition: all 0.5s;
  cursor: pointer;
  user-select: none; 
}

.dropdown__amount-minus:hover,
.dropdown__amount-plus:hover {
  background-color: #1ec7c2;
  color: #fff;
}

.dropdown__amount-number {
  padding: 0 10px;
  display: flex;
  align-items: center;
  justify-content: center;
  min-width: 40px;
  user-select: none; 
}

.checkbox {
  position: relative;
  margin: 0;
  text-align: left;
}

.checkbox label {
  cursor: pointer;
  display: inline;
  line-height: 1.25em;
  vertical-align: top;
  clear: both;
  padding-left: 1px;
  font-size: 13px;
  color: #d3d1df;
  user-select: none; 
}
.checkbox label:not(:empty) {
  padding-left: 0.75em;
}
.checkbox label:before, .checkbox label:after {
  content: "";
  position: absolute;
  left: 0;
  top: 0;
}
.checkbox label:before {
  width: 1.25em;
  height: 1.25em;
/*   background: #fff; */
  border: 1px solid rgba(255, 255, 255, 0.54);
  border-radius: 0.325em;
  cursor: pointer;
  transition: background .3s;
}
.checkbox input[type="checkbox"] {
  outline: 0;
  visibility: hidden;
  width: 1.25em;
  margin: 0;
  display: block;
  float: left;
  font-size: inherit;
}
.checkbox input[type="checkbox"]:checked + label:before {
  background: #1ec7c2;
  border: none;
}
.checkbox input[type="checkbox"]:checked + label:after {
  transform: translate(0.25em, 0.3365384615em) rotate(-45deg);
  width: 0.75em;
  height: 0.375em;
  border: 0.125em solid #fff;
  border-top-style: none;
  border-right-style: none;
}
.checkbox input[type="checkbox"]:disabled + label:before {
  border-color: rgba(255, 255, 255, 0.26);
}
.checkbox input[type="checkbox"]:disabled:checked + label:before {
  background: rgba(255, 255, 255, 0.26);
}

$(document).ready(function() {
  // DROPLIST
  $(".dropdown__select").on("click", function() {
    $(this).toggleClass("active");
    $(".dropdown__list").slideToggle();
  });

  // PLUS MINUS BUTTONS
  $(".dropdown__amount-minus").on("click", function() {
    var i = $(this)
      .siblings(".dropdown__amount-number")
      .text();
    if (i > 0) {
      $(this)
        .siblings(".dropdown__amount-number")
        .text(--i);
    }
    count();
  });

  $(".dropdown__amount-plus").on("click", function() {
    var i = $(this)
      .siblings(".dropdown__amount-number")
      .text();
    $(this)
      .siblings(".dropdown__amount-number")
      .text(++i);
    count();
  });

  // CHECKBOX
  $(".checkbox").on("click", function() {
    count();
  });

  // ADDING INFO TO SELECT
  function count() {
    var count = $(".dropdown__amount-number");
    var amount = 0;
    count.each(function(i, el) {amount += +$(el).text();});
     if ($("#business").is(":checked")) {
      $(".dropdown__select").text(amount + " " + declOfNum(amount, ["пассажир", "пассажира", "пассажиров"]) + ", Бизнес класс");
    } else {
       $(".dropdown__select").text(amount + " " + declOfNum(amount, ["пассажир", "пассажира", "пассажиров"]) + ", Эконом");
    }
    
  }

  // DECLENISIONS
  function declOfNum(number, titles) {
    cases = [2, 0, 1, 1, 1, 2];
    return titles[
      number % 100 > 4 && number % 100 < 20
        ? 2
        : cases[number % 10 < 5 ? number % 10 : 5]
    ];
  }

  count();
});