20 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 3 3 5 5 7 7 9 9 11 11 13 13 15 15 17 17 19 0 0 * 654729075 perfect matchings on 20 nodes, Knuth V4a p. 343