2084: Famil Door and Roads
Time Limit: 2 Sec Memory Limit: 256 MBSubmit: 0 Solved: 0
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Description
Famil Door’s City map looks like a tree (undirected connected acyclic graph) so other people call it Treeland. There are n intersections in the city connected by n-1 bidirectional roads.
There are m friends of Famil Door living in the city. The i-th friend lives at the intersection ui and works at the intersection vi. Everyone in the city is unhappy because there is exactly one simple path between their home and work.
Famil Door plans to construct exactly one new road and he will randomly choose one among n·(n-1)/2 possibilities. Note, that he may even build a new road between two cities that are already connected by one.
He knows, that each of his friends will become happy, if after Famil Door constructs a new road there is a path from this friend home to work and back that doesn't visit the same road twice. Formally, there is a simple cycle containing both ui and vi.
Moreover, if the friend becomes happy, his pleasure is equal to the length of such path (it's easy to see that it's unique). For each of his friends Famil Door wants to know his expected pleasure, that is the expected length of the cycle containing both ui and vi if we consider only cases when such a cycle exists.
The first line of the input contains integers n and m (2≤n, m≤100000)− the number of the intersections in the Treeland and the number of Famil Door's friends.
Then follow n-1 lines describing bidirectional roads. Each of them contains two integers ai and bi (1≤ai,bi≤n)− the indices of intersections connected by the i-th road.
Last m lines of the input describe Famil Door's friends. The i-th of these lines contain two integers ui and vi (1≤ui,vi≤n,ui≠vi)− indices of intersections where the i-th friend lives and works.
For each friend you should print the expected value of pleasure if he will be happy. Your answer will be considered correct if its absolute or relative error does not exceed 10-6.
Namely: let's assume that your answer is a, and the answer of the jury is b. The checker program will consider your answer correct, if 
.
4 3
2 4
4 1
3 2
3 1
2 3
4 1
4.00000000
3.00000000
3.00000000
3 3
1 2
1 3
1 2
1 3
2 3
2.50000000
2.50000000
3.00000000
Consider the second sample.
- Both roads (1,2) and (2,3) work, so the expected length if
- Roads (1,3) and (2,3) make the second friend happy. Same as for friend 1 the answer is 2.5
- The only way to make the third friend happy is to add road (2,3), so the answer is 3