如何在CSS中选择换行flex行的最后一项

<div class="flex-container">
  <div class="flex-item">Item 1</div>
  <div class="flex-item">Item 2</div>
  <div class="flex-item">Item 3</div>
  <div class="flex-item">Item 4</div>
  <div class="flex-item">Item 5</div>
  <div class="flex-item">Item 6</div>
</div>

.flex-container {
  display: flex;
  flex-wrap: wrap;
}

.flex-item {
  width: 100px;
  height: 100px;
  background-color: #ccc;
  margin: 10px;
}

.flex-item:last-child {
  background-color: red;
}