JS跨域的问题！！！高分求助！！！分不够再加！！！

http://a2.net.com/main.htm:
<iframe name=abc src="http://a1.net.com/ttt.htm"></iframe>
<input type="text" name="bbb" value="0" />


http://a1.net.com/ttt.htm:
<input type=button value=ok onclick="aaa();">
<SCRIPT LANGUAGE="JavaScript">
var i=0;
function aaa()
{
parent.document.getElementById("abc").height=i*30;
parent.document.getElementById("bbb").value=i;
i++;
}
</SCRIPT>