var a = parseInt(window.document.form1.vip_price1.value)
if(isNaN(a)) a = 0var b = parseInt(window.document.form1.vip_price2.value)
if(isNaN(b)) b = 0
window.document.form1.vip_price.value=a+b
if(isNaN(a)) a = 0var b = parseInt(window.document.form1.vip_price2.value)
if(isNaN(b)) b = 0
window.document.form1.vip_price.value=a+b
<HTML>
<HEAD>
<TITLE> New</TITLE>
<script>
function tt()
{
a3.value=parseInt(a1.value) + parseInt(a2.value)
}
</script>
</HEAD><BODY>
<input type="text" id="a1"><br>
<input type="text" id="a2"><br>
<input type="text" id="a3"><br>
<input type="button" onclick="tt()" value="Click me">
</BODY>
</HTML>