" ": 1 - ( 1 6) 2 - ( 2 6)... 8 - ( 4, 5, 6, 15) 9 - ( 7, 8, 9 15) 10 - " " ( 10, 11, 12 15) 11 - ( 13, 14, 15 15) 12 - 1 6 13 - 2 6