We were waiting several weeks to setting this contest and hope the problem was good enough.
Events
There was a difficulty with 805E - Ice cream coloring/804C - Ice cream coloring. A little bug in the checker fortunately yields accepting an incorrect solution of only one person during the contest. I should apologize all of you because of this.
For this sentence, Vertices which have the i-th (1 ≤ i ≤ m) type of ice cream form a connected subgraph.
You can find the meaning of "connected subgraph" with connected and subgraph, thus it can be empty as it is more logical. How ever, I should apologize all of the participants because of weak sample tests in the statement.
I will write the full editorial in the few next days, now some hints and short solutions exist here.
805C - Find Amir / 804A - Find Amir
805D - Minimum number of steps / 804B - Minimum number of steps
805E - Ice cream coloring / 804C - Ice cream coloring
805F - Expected diameter of a tree / 804D - Expected diameter of a tree