{18: 1, 20: 1}
19 20
{18: 2, 20: 1, 15: 2, 19: 2}
13 15
{18: 2, 20: 1, 15: 2, 19: 2, 11: 2, 13: 2}
14 15
{18: 2, 20: 1, 15: 2, 19: 2, 11: 3, 13: 3, 4: 2, 14: 3}
{18: 3, 20: 2, 15: 3, 19: 3, 11: 3, 13: 4, 4: 2, 14: 4, 12: 4}
4