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<style>
.youtube {
  position: relative;
  width: 100%;
  padding-top: 56.25%;
}
.youtube iframe {
  position: absolute;
  top: 0;
  right: 0;
  width: 100% !important;
  height: 100% !important;
}
</style>

<div class="youtube">
  <iframe src="https://www.youtube.com/embed/R1_Qo-MqFc4?rel=0" width="100%" frameborder="0" allowfullscreen="allowfullscreen"></iframe>
</div>