#include<bits/stdc++.h>
using namespace std;
int main(){
    int i,n,a[21],c=0,max1=0,m;
    float max=0;
    cin>>n>>m;
    for(i=1;i<=n;i++){
        cin>>a[i];
    }
    for(i=1;i<=n-m+1;i++){
        c=0;
        for(int j=i;j<=i+m-1;j++){
            c+=a[j];
        }
        if(max<(c*1.00/m)){
            max=c*1.00/m;
        }
    }
    printf("%.2f",max);
    return 0;
}
/**************************************************************
	Problem: 1165
	User: lijinkai
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/