<ul class="nav">
  <li class="nav-item"><a href="#">Item 1</a></li>
  <li class="nav-item"><a href="#">Item 2</a></li>
  <li class="nav-item"><a href="#">Item 3</a></li>
  <li class="nav-item"><a href="#">Item 4</a></li>
  <li class="nav-item"><a href="#">Item 5</a></li>
</ul>

.nav {

  background: #ED4343;
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
  list-style: none;

}
.nav-item {

  flex: 1;
  list-style: none;
  text-align: center;

}

a {

  display: block;
  padding: 20px;
  border: 1px solid #FFFFFF;
  color: #FFFFFF;
  text-decoration: none

}