CF963E.Circles of Waiting
NOI/NOI+/CTSC
通过率:0%
时间限制:2.00s
内存限制:256MB
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题目描述
A chip was placed on a field with coordinate system onto point (0, 0).
Every second the chip moves randomly. If the chip is currently at a point (x, y), after a second it moves to the point (x - 1, y) with probability _p_1, to the point (x, y - 1) with probability _p_2, to the point (x + 1, y) with probability _p_3 and to the point (x, y + 1) with probability _p_4. It's guaranteed that _p_1 + _p_2 + _p_3 + _p_4 = 1. The moves are independent.
Find out the expected time after which chip will move away from origin at a distance greater than R (i.e.
will be satisfied).
一枚芯片被放置在带有坐标系的平面上的点 (0,0) 处。
每一秒,该芯片随机移动一次。若芯片当前位于点 (x,y),则经过一秒后,它将以概率 p1 移动到点 (x−1,y),以概率 p2 移动到点 (x,y−1),以概率 p3 移动到点 (x+1,y),以概率 p4 移动到点 (x,y+1)。已知 p1+p2+p3+p4=1。各次移动相互独立。
求芯片首次离开原点距离超过 R(即满足
)的期望时间。
输入格式
First line contains five integers R, _a_1, _a_2, _a_3 and _a_4 (0 ≤ R ≤ 50, 1 ≤ _a_1, _a_2, _a_3, _a_4 ≤ 1000).
Probabilities p__i can be calculated using formula
.
第一行包含五个整数 R、a1、a2、a3 和 a4(其中 0≤R≤50,1≤a1,a2,a3,a4≤1000)。
概率 pi 可通过公式
计算。
输出格式
It can be shown that answer for this problem is always a rational number of form
, where
.
Print P·Q - 1 modulo 109 + 7.
可以证明,本题的答案总是一个形如
的有理数,其中
。
请输出 $ P \cdot Q^{-1} \bmod (10^9 + 7) $。
输入输出样例
输入#1
0 1 1 1 1
输出#1
1
输入#2
1 1 1 1 1
输出#2
666666674
输入#3
1 1 2 1 2
输出#3
538461545
说明/提示
In the first example initially the chip is located at a distance 0 from origin. In one second chip will move to distance 1 is some direction, so distance to origin will become 1.
Answers to the second and the third tests:
and
.
在第一个例子中,芯片初始时位于距原点距离为 0 的位置。一秒后,芯片将向某个方向移动至距原点距离为 1 的位置,因此其到原点的距离变为 1。
第二和第三个测试用例的答案分别为:
和
。
输入解题思路,AI测评打分。不知道怎么写?