 .main {
  display: flex;
  border: 1px solid black;
  flex-wrap: wrap;
  flex-direction: row;
  width: 400px;
}
  
.main > div {
  height: 50px;   
  width: 50px;
}
 
div:nth-child(even){
  background-color:black;
}

	.main {
	display: flex;
	border: 1px solid black;
	flex-wrap: wrap;
	flex-direction: row;
	width: 400px;
	}

	.main > div {
	height: 50px;
	width: 50px;
	}

	div:nth-child(even){
	background-color:black;
	}