如何实现第二个子盒子竖直占满父盒子？

具体问题如上图所示，绿色盒子的高度我需要恰好从红色子盒子下部到父盒子底部，我的需求不能用overflow：hidden等需要恰好的高度

flex布局http://www.ruanyifeng.com/blog/2015/07/flex-grammar.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box{
          display: flex;
          flex-direction: column;
          border: 1px solid green;
          height:500px;
    }
    .son1{
      height:auto;
      border: 1px solid red;
    }
    .son2{
      flex:1;
      background: red;
    }
  </style>
</head>
<body>
 <div class="box">
   <div class="son1">55</div>
   <div class="son2"></div>
 </div>
</body>
<script>

</script>
</html>

用flex就好