关于前端ul li怎么平均分布

请问怎么把一个ul最为一个盒子，然后里面有几个li,然后这几个li在ul里面纵向排列，并且平均分布，每一个的间隙相同，并且每个li里面还有两个span两端分布，做了很久一直做不出来，才来麻烦各位，万分感谢！

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<!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>
        ul, li {
            list-style-type: none;
            margin: 0;
            padding: 0;
        }
        .s-ul {
            border: 1px solid #000;
            padding: 10px;
            margin-bottom: -10px;
            display: inline-block;
        }
        .s-ul li {
            margin-right: -10px;
        }
        .s-ul li:last-child {
            margin-bottom: -10px;
        }
        .s-ul li span {
            display: inline-block;
            width: 40px;
            height: 20px;
            margin-right: 10px;
            border: 1px solid #000;
            vertical-align: top;
        }
        .s-ul li span:first-child {
            margin-right: 10px;
            margin-bottom: 10px;
        }
    </style>
</head>
<body>
    <ul class="s-ul">
        <li class="s-li">
            <span></span>
            <span></span>
        </li>
        <li class="s-li">
            <span></span>
            <span></span>
        </li>
        <li class="s-li">
            <span></span>
            <span></span>
        </li>
    </ul>
</body>
</html>