bootstrap(响应式)加减输入框

<div class="row">

 <div class="col-lg-6">

   <div class="input-group">

     <span class="input-group-btn">

       <button class="btn btn-default" type="button">-</button>

     </span>

     <input type="text" class="form-control" placeholder="2">

     <span class="input-group-btn">

       <button class="btn btn-default" type="button">+</button>

     </span>

   </div>


 </div>


</div>