```
function CalculateSum()
{
var Form = document.frmsum;
var txtbx1= Form.txt1.value;
var txtbx2 = Form.txt2.value;
var txtbx3 = Form.txt3.value;
var txtbx4 = Form.txt4.value;
var allsum = Number(txtbx1)+Number(txtbx2)+Number(txtbx3)+Number(txtbx4);
if(!isNaN(allsum) && allsum !=Infinity)
{
Form.txtsum.value = allsum;
}
return CheckZero()
}
```

on page load

`txtsum.Attributes.Add("onblur", "return CalculateSum()");`

Thanks

Ashish