function isNumberKey(evt)
      {
         var charCode = (evt.which) ? evt.which : event.keyCode
         if (charCode > 31 && (charCode < 48 || charCode > 57))
            return false;

         return true;
      }
function sal()
{
var bs = document.form1.bs.value;
var pf = document.form1.pf.value;
var da = document.form1.da.value;
var hra = document.form1.hra.value;
var ai = document.form1.ai.value;
var pt = document.form1.pt.value;
var og = document.form1.og.value;
if((bs=="" && pf=="" && da=="" && hra=="" && ai=="" && pt=="" && og=="")||(bs=="" || pf=="" || da=="" || hra=="" || ai=="" || pt=="" || og==""))
{
alert("please enter the value for all fields");
}
else
{
var a = (eval(bs)+eval(da)+eval(hra)+eval(ai));
var b = (eval(pf)+eval(pt)+eval(og));
document.form1.mst.value=a-b;

}
}