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