Flexbox grid

HTML

<div class="container">
    <div class="item">1</div>
    <div class="item">2</div>
    <div class="item">3</div>
    <div class="item">4</div>
    <div class="item">5</div>
</div>

CSS

.container {
  display: flex;
  flex-direction: column;
  width: 100%;
  height: 300px;
  flex-wrap: wrap;
}

.item {
  border: 2px solid #ddd;
  padding: 20px;
  background: grey;
  height: 50%;
  width: 25%;
  box-sizing: border-box;
}


.item:nth-child(1) {
  width: 50%;
  height: 100%;
  
}