html - Dynamic sidebar width depending on the main div -

i have following code

<div id="content">     <div id="sidebar"></div>     <div id="main"></div> </div> 

and css

 #main{      width: 840px;      margin: 0 auto;  }   #sidebar{      float: left;      width: ?; -> need fill dhe auto space  } 

the problem how can make #sidebar div fill space left main div? image shows want

you can use

 #sidebar{      float: left;      width: calc(100% - 840px);  }