serialization-maxJsonLength web.config error

I have a problem when serializing a list<> of c# objects.

when i call a webservice via .ajax there is an error saying that that the length has excedeed..

to avoid this error ,i changed my web.config by adding in the begining of as bellow:

       <configuration>

         <system.web.extensions>
           <scripting>
             <webServices>
                <jsonSerialization maxJsonLength="开发者_运维百科500000"></jsonSerialization>
             </webServices>
        </scripting>
        </system.web.extensions>
        ...
      </configuration>

now i'm getting an error when i launch my asp.net application "system.web.extensions unknown"?

  <system.web.extensions>
    <scripting>
      <scriptResourceHandler enableCompression="true" enableCaching="true"/>
      <webServices>
        <jsonSerialization maxJsonLength="50000000"/>
      </webServices>
    </scripting>
  </system.web.extensions>

This works for me. The tag orders are wrong.