*{margin:0;padding:0;box-sizing: border-box;}
.box{
  border:5px solid red;
  height:100vh;
  display: flex;
  flex-direction: column;
}
.head{
  border:5px solid green;
  flex: none;
  height:80px;
}
.main{
 border:5px solid blue;
  flex:1;
  overflow-y: auto;
}
p{
line-height:50px;
}