C. Anadi and Domino 《 Codeforces Round #588 (Div. 2) 》
Anadi has a set of dominoes. Every domino has two parts, and each part contains some dots. For every a a and b b such that 1 ≤ a ≤ b ≤ 6 1≤a≤b≤6, there is exactly one domino with a a dots on one half and b b dots on the other half. The set contains exactly 21 21 dominoes. Here is an exact illustration of his set: Also, Anadi has an undirected graph without self-loops and multiple edges. He wants to choose some dominoes and place them on the edges of this graph. He can use at most one domino of each type. Each edge can fit at most one domino. It's not necessary to place a domino on each edge of