输入：

1 0 0 0 0 0 0

输出：

0.000

错误题解：

输出nan：

https://www.luogu.com.cn/blog/Alansp/solution-p3802

https://www.luogu.com.cn/blog/codecodeakioi/solution-p3802

https://www.luogu.com.cn/blog/rainybunny/solution-p3802

https://www.luogu.com.cn/blog/138812/solution-p3802

https://www.luogu.com.cn/blog/1226ray/solution-p3802

https://www.luogu.com.cn/blog/ButterflyDew/solution-p3802

https://www.luogu.com.cn/blog/user30361/solution-p3802

https://www.luogu.com.cn/blog/orangebird/solution-p3802

输出-0.000（话说这算错吗）：

https://www.luogu.com.cn/blog/user47641/solution-p3802