CF808B.Average Sleep Time

It's been almost a week since Polycarp couldn't get rid of insomnia. And as you may already know, one week in Berland lasts k days!

When Polycarp went to a doctor with his problem, the doctor asked him about his sleeping schedule (more specifically, the average amount of hours of sleep per week). Luckily, Polycarp kept records of sleep times for the last n days. So now he has a sequence _a_1, _a_2, ..., a__n, where a__i is the sleep time on the i-th day.

The number of records is so large that Polycarp is unable to calculate the average value by himself. Thus he is asking you to help him with the calculations. To get the average Polycarp is going to consider k consecutive days as a week. So there will be n - k + 1 weeks to take into consideration. For example, if k = 2, n = 3 and a = [3, 4, 7], then the result is .

You should write a program which will calculate average sleep times of Polycarp over all weeks.

自从波利卡普无法摆脱失眠已经快一周了！而正如你可能已经知道的那样，贝尔兰的一周持续 $k$ 天！

当波利卡普带着他的问题去看医生时，医生询问了他的睡眠作息（更具体地说，是每周平均睡眠小时数）。幸运的是，波利卡普记录了最近 $n$ 天的睡眠时间。因此，他现在拥有序列 $a_1,\ a_2,\ \dots,\ a_n$，其中 $a_i$ 表示第 $i$ 天的睡眠时间。

由于记录数量过于庞大，波利卡普无法自行计算平均值。因此，他请你帮忙完成计算。为了求得平均值，波利卡普将把任意连续 $k$ 天视为一周。因此，需要考虑的周数共有 $n - k + 1$ 周。例如，若 $k = 2$，$n = 3$，且 $a = [3,\ 4,\ 7]$，则结果为 。

你需要编写一个程序，计算波利卡普在所有这些周内的平均睡眠时间。

The first line contains two integer numbers n and k (1 ≤ k ≤ n ≤ 2·105).

The second line contains n integer numbers _a_1, _a_2, ..., a__n (1 ≤ a__i ≤ 105).

第一行包含两个整数 $n$ 和 $k$（$1 ≤ k ≤ n ≤ 2·10^5$）。

第二行包含 $n$ 个整数 $a_1, a_2, ..., a_n$（$1 ≤ a_i ≤ 10^5$）。

Output average sleeping time over all weeks.

The answer is considered to be correct if its absolute or relative error does not exceed 10 - 6. In particular, it is enough to output real number with at least 6 digits after the decimal point.

输出所有周的平均睡眠时间。

若答案的绝对误差或相对误差不超过 $10^{-6}$，则视为正确。特别地，输出至少保留小数点后 6 位的实数即可。

In the third example there are n - k + 1 = 7 weeks, so the answer is sums of all weeks divided by 7.

在第三个例子中，共有 $n - k + 1 = 7$ 周，因此答案为所有周的和除以 7。