1: ( 1,20) ( 2,19) ( 3,18) ( 4,17) ( 5,16) ( 6,15) ( 7,14) ( 8,13) ( 9,12) (10,11) (11,10) (12, 9) (13, 8) (14, 7) (15, 6) (16, 5) (17, 4) (18, 3) (19, 2) (20, 1)