Get value from dataset !

Hi Everyone !

I have trouble with dataset, I need help !
            In my SQL query only display one value last ID,
            How do get this value to dataset ?
            I used ds.Tables[0].Rows[0].ToString() or ds.Tables[0].Columns[0].ToString() but it was error !
            How do I get it ?

ds = new DataSet();
            da = new SqlDataAdapter("SELECT MAX(PostID) AS LastID FROM POSTs')", con);
            da.Fill(ds);