function numchange(obj) {
  var txt = document.getElementById(obj).value;
  if (makenumeric(txt)!=txt) document.getElementById(obj).value = makenumeric(txt);
}

function makenumeric(sText) {
   var ValidChars = "0123456789";
   var myreturn="";
   var Char;

   for (i = 0; i < sText.length; i++) {
	  Char = sText.charAt(i);

	  if ((i==0 && Char=='-') || (ValidChars.indexOf(Char) != -1)) {
		  myreturn = myreturn + Char;
	  }
   }
   return myreturn;
}