Update list item with itemadded event receivers sp2013

I have an itemadded event receivers for a List, my ItemAdded function here:

public override void ItemAdded(SPItemEventProperties properties)
        {
            base.ItemAdded(properties);
            SPListItem item = properties.ListItem;
            string a= item["a"].ToString();
            string b= item["b"].ToString().Trim();
// Sure that 'b' is empty
            if (String.IsNullOrEmpty(b))
            {
                item["b"] = a;
                item.Update();
            }

        }

But nothing happen after I added the item??? I'm a new in Event Receiver