[(0,1),(0,2),(1,3),(3,4),(1,5),(4,6),(2,7),(6,7),(6,8),(4,9),(5,10),(8,11),(9,12),(10,13),(5,14),(2,15),(7,16),(3,17),(10,18),(9,19),(8,20)]
[(0,1),(1,2),(1,3),(2,4),(3,4),(2,5),(5,6),(6,7),(4,8),(8,9),(5,10),(10,11),(7,12),(10,13),(9,14),(3,15),(8,16),(7,17),(6,18),(0,19),(9,20)]
