In FCKedtior how to count Character using javascript Real-

following is my code
now I want to realise such a function that can count character real-time we have entered using javascript .........
please help me .......thank you>>>>>>
<table width="100%" border="0">
<tr>

</tr>
<tr>
<td>单位简介最少20字，最长2000字，属于必填项</td>
</tr>
<tr>
<td>此页面打开后20分钟过期，请尽快填写。如果在20分钟内无法完成，请在word起草后，粘贴到这里</td>
</tr>
</table>
<hr />
<div style="border:#0099CC 2px solid; background-color:#CCCCCC ;width:100%" onkeypress="test();">
<html:form action="/edit" method="post" onsubmit="return chknext()">
<html:hidden property="method" value="editInfo" />
<%fckEditor.setValue("");
out.println(fckEditor);
%>

<br />
<html:submit property="submit" value="下一步" style="width:100px; height:40px"></html:submit>
<span id="num">已输入字符<span>
</html:form>
</div>