ul{
            list-style: none;
        }
        #container{
            position: relative;
            width: 400px;
            height: 200px;
            margin: 20px auto;
        }
        
        .pic li {
            position: absolute;
            top: 0;
            left: 0;
            display: none;
        }
        .pic li img {
            width: 400px;
            height: 200px;
        }
        #position{
            position: absolute;
            bottom: 0;
            right:0;
            margin: 0;
            background: #000;
            opacity: 0.4;
            width: 400px;
            text-align: center;
        }
        #position li{
            width: 10px;
            height: 10px;
            margin:0 2px;
            display: inline-block;
            -webkit-border-radius: 5px;
            border-radius: 5px;
            background-color: #afafaf;
        }
        #position .cur{
            background-color: #ff0000;
        }

         .arrow { 
             cursor: pointer;
              display: none; 
              line-height: 39px; 
              text-align: center; 
              font-size: 36px; 
              font-weight: bold; 
              width: 40px; 
              height: 40px;  
              position: absolute; 
              z-index: 2; 
              top: 50%;
              margin-top: -20px; /*width的一半*/
              background-color: RGBA(0,0,0,.3); 
              color: #fff;
          }
        .arrow:hover { 
            background-color: RGBA(0,0,0,.7);
        }
        #container:hover .arrow { 
            display: block;
        }
        #prev { 
            left: 20px;
        }
        #next { 
            right: 20px;
        }