Calculate Sum on aspx page, then insert figure on aspx.vb page

function fnCalculateSum()
{
var result = 0;
if(document.getElementById("<%=Textbox1.ClientID%>"))
{
result = result + parseInt(document.getElementById("<%=Textbox1.ClientID%>").value);
}
if(document.getElementById("<%=Textbox2.ClientID%>"))
{
result = result + parseInt(document.getElementById("<%=Textbox2.ClientID%>").value);

}
document.getElementById("<%=Textbox9.ClientID%>").value = result;

}
</script>

I am using above code in my aspx page to sum textbox1 and textbox2 and show the calculated sum in textbox9.  But on aspx.vb page, if i try to do a insert into sqldatabase the TextBox9.Text  is empty here.   I must be missing something that brings the value from 1st page into code behind.