function CheckForm(f){
  var eles=new Array("username","password");
  var msgs=new Array("Please, enter your username","Please, enter your password");
  for(i=0;i<eles.length;i++){
     if(f.elements[eles[i]].value==''){
        alert(msgs[i]);
        f.elements[eles[i]].focus();
        return false;
     }
  }
  return true;
}