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A11705 | Circles of Waiting

时间限制1s
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. ![](/uploads/acgo/image/a4f65975d5b2e035_7e7edc37560e.jpeg) will be satisfied).

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 ![](/uploads/acgo/image/6c354378cf839d00_05a3cd624505.jpeg).

输出格式

It can be shown that answer for this problem is always a rational number of form ![](/uploads/luogu/CF963E/ed2c29110477df816fcacba17d99273426c4dd91_8a08a8c4c2e0.png), where ![](/uploads/acgo/image/94dd50fe5b34857f_c99e4025f386.jpeg).

Print $P·Q^{-1}$ modulo $10^{9}+7$ .

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
C++ 编辑器
输入
输出