題目：
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 a1, a2, ..., an, where ai 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.
The first line contains two integer numbers n and k (1 ≤ k ≤ n ≤ 2·105).
The second line contains n integer numbers a1, a2, ..., an (1 ≤ ai ≤ 105).
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.
3 2 3 4 7
9.0000000000
1 1 10
10.0000000000
8 2 1 2 4 100000 123 456 789 1
28964.2857142857
比較簡單的一個題，昨晚打virtue contest 卻死活沒寫出來，然後關機睡覺。
其實就是一個模擬嘛。sum[i]：=前i項和。
res+=(sum[i]-sum[j])/weeks(ps,i可以寫成j+k無本質區別)
放在這兒，提醒自己基本功不能丟。
code:
#include<cstdio>
typedef long long LL;
const int MAXN=2e5+5;
LL sum[MAXN];
int main(){
int n,k;scanf("%d%d",&n,&k);
double divide=(double)1.0*n-k+1;
for(int i=1;i<=n;++i){
int a;scanf("%d",&a);
sum[i]=sum[i-1]+a;
}
double res=0.0;
int i=0,j=k;
for(;j<=n;){
res+=(sum[j]-sum[i])/divide;
++j;++i;
}
printf("%.8f\n",res);
}