试试

*{
    margin: 0;
    padding: 0;
}
#wrapper{
    width: 100%;
    overflow: hidden;
}
ul{
    margin-left: 50%;
    width: 2100px;
    display: inline-block;
    background-color: gold;
    font-size: 0;
    /*overflow: hidden;*/
}
li{
    /*float: left;*/
    display: inline-block;
    width: 700px;
    height: 100px;    
}
li:nth-child(1){
    margin-left: -50%;
}
//
<p id="wrapper">
   <ul>
       <li style="background: #000000">1</li>
       <li style="background: #FF5858">2</li>
       <li style="background: #000000">N</li>
   </ul>
</p>