I have built a brute force solution to this problem but of course time limit exceeded monster has appeared!

please can anyone explain a solution or just give a hint for this problem, i have read the editorial but i didn't understand what is written.

Problem B

recently i have been studying graph theory from competitive programming book ,in applications of DFS there was articulation points and bridges . He defines other than visited array a another one here is the statement.

[This algorithm maintains two numbers: dfs_num(u) and dfs_low(u). Here, dfs_num(u) stores the iteration counter when the vertex u is visited for the first time and not just for distinguishing DFS_WHITE versus DFS_GRAY/DFS_BLACK. The other number dfs_low(u) stores the lowest dfs_num reachable from DFS spanning sub tree of u. Initially dfs_low(u) = dfs_num(u) when vertex u is first visited. Then, dfs_low(u) can only be made smaller if there is a cycle (some back edges exist). Note that we do not update dfs_low(u) with back edge (u, v) if v is a direct parent of u. ]

please anyone give an explanation for this part.