Codeforces Round 487 (Div. 2)
E
A Trance of Nightfall
Codeforces
2700
原题
← D→
翻译中…
  • The cool breeze blows gently, the flowing water ripples steadily."Flowing and passing like this, the water isn't gone ultimately; Waxing and waning like that, the moon doesn't shrink or grow eventually.""Everything is transient in a way and perennial in another."Kanno doesn't seem to make much sense out of Mino's isolated words, but maybe it's time that they enjoy the gentle breeze and the night sky — the inexhaustible gifts from nature.Gazing into the sky of stars, Kanno indulges in a night's tranquil dreams. *
翻译中…

There is a set $S$ of $n$ points on a coordinate plane.

翻译中…

Kanno starts from a point $P$ that can be chosen on the plane. $P$ is not added to $S$ if it doesn't belong to $S$. Then the following sequence of operations (altogether called a move) is repeated several times, in the given order:

翻译中…
  1. Choose a line $l$ such that it passes through at least two elements in $S$ and passes through Kanno's current position. If there are multiple such lines, one is chosen equiprobably.
  2. Move to one of the points that belong to $S$ and lie on $l$. The destination is chosen equiprobably among all possible ones, including Kanno's current position (if it does belong to $S$).
翻译中…

There are $q$ queries each consisting of two integers $(t_i, m_i)$. For each query, you're to help Kanno maximize the probability of the stopping position being the $t_i$-th element in $S$ after $m_i$ moves with a proper selection of $P$, and output this maximum probability. Note that according to rule 1, $P$ should belong to at least one line that passes through at least two points from $S$.

翻译中…

Input

翻译中…

The first line contains a positive integer $n$ ($2 \leq n \leq 200$) — the number of points in $S$.

翻译中…

The $i$-th of the following $n$ lines contains two space-separated integers $x_i$ and $y_i$ ($-10^4 \leq x_i, y_i \leq 10^4$) — the coordinates of the $i$-th point in $S$. The input guarantees that for all $1 \leq i \lt j \leq n$, $(x_i, y_i) \neq (x_j, y_j)$ holds.

翻译中…

The next line contains a positive integer $q$ ($1 \leq q \leq 200$) — the number of queries.

翻译中…

Each of the following $q$ lines contains two space-separated integers $t$ and $m$ ($1 \leq t_i \leq n$, $1 \leq m_i \leq 10^4$) — the index of the target point and the number of moves, respectively.

翻译中…

Output

翻译中…

Output $q$ lines each containing a decimal number — the $i$-th among them denotes the maximum probability of staying on the $t_i$-th point after $m_i$ steps, with a proper choice of starting position $P$.

翻译中…

Your answer will be considered correct if each number in your output differs from the corresponding one in jury's answer by at most $10^{-6}$.

翻译中…

Formally, let your answer be $a$, and the jury's answer be $b$. Your answer is considered correct if $|a - b| \le 10^{-6}$.

翻译中…

Note

翻译中…

The points in $S$ and possible candidates for line $l$ are depicted in the following figure.

翻译中…

image

翻译中…

For the first query, when $P = (-1, -3)$, $l$ is uniquely determined to be $3x = y$, and thus Kanno will move to $(0, 0)$ with a probability of $\frac 1 2$.

翻译中…

For the third query, when $P = (2, 2)$, $l$ is chosen equiprobably between $x + y = 4$ and $x = y$. Kanno will then move to the other four points with a probability of $\frac 1 2 \cdot \frac 1 3 = \frac 1 6$ each, or stay at $(2, 2)$ with a probability of $\frac 1 3$.

翻译中…

样例

Input #1
5
0 0
1 3
2 2
3 1
4 4
10
1 1
2 1
3 1
4 1
5 1
3 2
3 3
3 4
3 5
3 6
Output #1
0.50000000000000000000
0.50000000000000000000
0.33333333333333331483
0.50000000000000000000
0.50000000000000000000
0.18518518518518517491
0.15226337448559670862
0.14494741655235482414
0.14332164812274550414
0.14296036624949901017
InputCopy OutputCopy
题目信息
题号
989E
时间限制
2 s
内存限制
256 MB
标签
动态规划
计算几何
矩阵
概率论

题面与样例版权归 Codeforces 及出题人所有;中文翻译由 AI 生成(CI 管线),如有出入请以原文为准。