# include <iostream>
#include <stdio.h>
# include <cstdio>
using namespace std;
int a[105];
int main()
{
freopen("temperatures.in","r",stdin);
freopen("temperatures.out","w",stdout);
int n,k,x,t;
int mins=10000000,maxs=-10000000;
cin>>n>>k;
int i;
for(i=1;i<=n;i++)
{
    cin>>x;
    a[i]=a[i-1]+x;
    if(i>=k)
    {
        t=a[i]-a[i-k];
        mins=min(mins,t);
        maxs=max(maxs,t);
    }
}
//cout<<maxs<<" "<<mins<<endl;
double p;
p=double((maxs-mins)/double(k));
//cout<<p<<endl;
printf("%.2f\n",p);
return 0;
}