function isNumberKey(e,evt) {
           var charCode;          
          if(e && e.which)
             { //if which property of event object is supported (NN4)
                e = e
                charCode = e.which //character code is contained in NN4's which property
             }
            else
                {
                    e = event
                    charCode = e.keyCode //character code is contained in IE's keyCode property
                }  
            //alert(charCode);
            if (charCode > 31 && (charCode < 48 || charCode > 57) && (charCode <40 || charCode > 41) && (charCode != 43) && (charCode <45 || charCode > 45))
            {  
                return false;
            }
            else
            {
             if(evt.value.length >0 && charCode == 43)
             {
              return false;
             }
             else
             {
              return true;
             }
            }
}