how to calculate the sum

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()
    }

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