.flex
{
  display: flex;
  flex-wrap: wrap;
	padding: 0;
	margin: 0;
	height: 100%;
	background: rgba(0,0,0,.2)
}

body, html {
margin: 0;
padding:0;
height: 100%;
}

.top {
 width: 100%;
 background: red;
}

.cls { background: blue; flex: 1 1 auto; border: 1px solid; }

.cls-2 { background: rgba(255,255,255,0.8); height: 100%; }