Shows Value Of Combobox In The Textbox

I have a Combobox and a textbox in my asp page. I want the value of Combobox in Onchange event shows in the textbox. this my code but it isnot correct.

<script Language="JavaScript">
function NavTo(a1)
{
document.forms[0].text1.value=a1
}
</Script>

<SELECT id=select1 name=select1 onchange=Navto(document.forms[0].select1.value)>
<OPTION value="1046">1046</OPTION>
<OPTION value="1045">1045</OPTION>
</select>

<INPUT type="text" id=text1 name=text1 >