Maze

一棵 $n$ 个点的树，起点和终点随机选取。运行如下程序， $V(x)$ 是与 $x$ 相邻的顶点列表，最初 $\mathrm{flag}$ 数组为 $\verb!false!$，DFS 的 参数是起点，你要求 $\mathrm{count}$ 的期望值。 $n≤ 10^5$。  翻译提供者：Zward_ToOoMmy

A maze is represented by a tree (an undirected graph, where exactly one way exists between each pair of vertices). In the maze the entrance vertex and the exit vertex are chosen with some probability. The exit from the maze is sought by Deep First Search. If there are several possible ways to move, the move is chosen equiprobably. Consider the following pseudo-code: ```plain DFS(x) if x == exit vertex then finish search flag[x] <- TRUE random shuffle the vertices' order in V(x) // here all permutations have equal probability to be chosen for i <- 1 to length[V] do if flag[V[i]] = FALSE then count++; DFS(y); count++; ``` $ V(x) $ is the list vertices adjacent to $ x $ . The $ flag $ array is initially filled as FALSE. $ DFS $ initially starts with a parameter of an entrance vertex. When the search is finished, variable $ count $ will contain the number of moves. Your task is to count the mathematical expectation of the number of moves one has to do to exit the maze.

The first line determines the number of vertices in the graph $ n $ ( $ 1<=n<=10^{5} $ ). The next $ n-1 $ lines contain pairs of integers $ a_{i} $ and $ b_{i} $ , which show the existence of an edge between $ a_{i} $ and $ b_{i} $ vertices ( $ 1<=a_{i},b_{i}<=n $ ). It is guaranteed that the given graph is a tree. Next $ n $ lines contain pairs of non-negative numbers $ x_{i} $ and $ y_{i} $ , which represent the probability of choosing the $ i $ -th vertex as an entrance and exit correspondingly. The probabilities to choose vertex $ i $ as an entrance and an exit equal  and  correspondingly. The sum of all $ x_{i} $ and the sum of all $ y_{i} $ are positive and do not exceed $ 10^{6} $ .

Print the expectation of the number of moves. The absolute or relative error should not exceed $ 10^{-9} $ .

2
1 2
0 1
1 0

1.00000000000000000000

3
1 2
1 3
1 0
0 2
0 3

2.00000000000000000000

7
1 2
1 3
2 4
2 5
3 6
3 7
1 1
1 1
1 1
1 1
1 1
1 1
1 1

4.04081632653

In the first sample the entrance vertex is always 1 and the exit vertex is always 2. In the second sample the entrance vertex is always 1 and the exit vertex with the probability of 2/5 will be 2 of with the probability if 3/5 will be 3. The mathematical expectations for the exit vertices 2 and 3 will be equal (symmetrical cases). During the first move one can go to the exit vertex with the probability of 0.5 or to go to a vertex that's not the exit vertex with the probability of 0.5. In the first case the number of moves equals 1, in the second one it equals 3. The total mathematical expectation is counted as $ 2/5×(1×0.5+3×0.5)+3/5×(1×0.5+3×0.5) $