Let $A,B,C,D,E,F$ be six teams in a sports competition, and each team will play exactly once with another team.
Now we know that Team $A,B,C,D,E$ had already played $5,4,3,2,1$ games, correspondingly.
So, how do I figure out which team haven't played a game with team $B$ yet?
This is to fulfill the 30 characters requirement.