<div class="swiper-container">
    <div class="swiper-wrapper">
      <div class="swiper-slide">Slide 1</div>
      <div class="swiper-slide">Slide 2</div>
      <div class="swiper-slide">Slide 3</div>
      <div class="swiper-slide">Slide 4</div>
      <div class="swiper-slide">Slide 5</div>
      <div class="swiper-slide">Slide 6</div>
      <div class="swiper-slide">Slide 7</div>
      <div class="swiper-slide">Slide 8</div>
      <div class="swiper-slide">Slide 9</div>
      <div class="swiper-slide">Slide 10</div>
      <div class="swiper-slide">Slide 11</div>
      <div class="swiper-slide">Slide 12</div>
      <div class="swiper-slide">Slide 13</div>
      <div class="swiper-slide">Slide 14</div>
      <div class="swiper-slide">Slide 15</div>
      <div class="swiper-slide">Slide 16</div>
      <div class="swiper-slide">Slide 17</div>
      <div class="swiper-slide">Slide 18</div>
      <div class="swiper-slide">Slide 19</div>
      <div class="swiper-slide">Slide 20</div>
    </div>
    <!-- Add Pagination -->
    <div class="swiper-pagination"></div>
    <div class="counter">
            </div>
  </div>

    html, body {
      position: relative;
      height: 100%;
    overflow: hidden;
    }
    body {
      background: #eee;
      font-family: Helvetica Neue, Helvetica, Arial, sans-serif;
      font-size: 14px;
      color:#000;
      margin: 0;
      padding: 0;

    }
    .swiper-container {
      width: 50%;
      height: 100%;
      margin-left: auto;
      margin-right: auto;
    }
    .swiper-slide {
      text-align: center;
      font-size: 18px;
      background: #fff;
      height: 100%;
    opacity: 0.3;

      /* Center slide text vertically */
      display: -webkit-box;
      display: -ms-flexbox;
      display: -webkit-flex;
      display: flex;
      -webkit-box-pack: center;
      -ms-flex-pack: center;
      -webkit-justify-content: center;
      justify-content: center;
      -webkit-box-align: center;
      -ms-flex-align: center;
      -webkit-align-items: center;
      align-items: center;
     transition: .3s all;
    }

.swiper-slide-active{
  opacity: 1;
  background-color: orange;
}


.counter {
  position: absolute;
  left: 5%;
  top: 5%;
  z-index: 100;
  font-size: 10vw;
  font-weight: bold;
  pointer-events: none;
  display: flex;
  justify-content: center;
  align-items: flex-start;
}

.counter__total {
  font-size: 0.5em;
  margin-top: .2em;
}

var swiper = new Swiper(".swiper-container", {
  slidesPerView: 1,
  loop: false,
  init: false,
  spaceBetween: 30,
  pagination: {
    el: ".swiper-pagination",
    clickable: true
  }
  
});

swiper.on("slideChange afterInit init", function () {
    let currentSlide = this.activeIndex + 1;
    document.querySelector('.counter').innerHTML = `
    <span class="counter__current">
    ${currentSlide < 10 ? '0' + currentSlide : currentSlide}
    </span> 
    / 
    <span class="counter__total">
      ${this.slides.length}
    </span>`;
});

swiper.init();