function change_visibility(id)
{
    if(document.getElementById(id).className == "hidden")
    {
        document.getElementById(id).className = "visible";
    }
    else
    {
        document.getElementById(id).className = "hidden";
    }
}

function checkNumber(e)
{
    if (!e) var e = window.event
    if (e.keyCode) code = e.keyCode;
    else if (e.which) code = e.which;

    if ((code < 48 || code > 57) && code != 8)
    {
      e.returnValue= false;
      e.cancelBubble = true;
      if (e.stopPropagation) e.stopPropagation();

      return false;
    }
    else
    {
      return true;
    }
}