其它
From $1$ to $30$, how many integers can be chosen at most to ensure that no two of chosen numbers sum to $36$? 
Overseas Competition->Knowledge Point->Combinatorics->Constructing and Proving
1, 2, 3, 4, 5, 18→ 6  $30-5-1=24$, $24\div2=12$  $$6+12=18$$ 