//什么时候我才能有杜爷一半强啊/kk
#include<cstdio>
#include<iostream>
#include<algorithm>
using namespace std;
const int N=2555;
int n,m,a[N];
double f[N][N];
int read(){
	char ch=getchar();int nn=0,ssss=1;
	while(ch<'0'||ch>'9'){if(ch=='-')ssss*=-1;ch=getchar();}
	while(ch>='0'&&ch<='9'){nn=nn*10+(ch-'0');ch=getchar();}
	return nn*ssss;
}
int main(){
	n=read();m=read();
	for(int i=1;i<=n;i++)a[i]=read();
	sort(a+1,a+n+1,[&](int a,int b){return a>b;});int sum=0;
	for(int i=1;i<=n;i++){
		sum+=a[i];
		for(int j=1;j<=m&&j<i;j++)
			f[i][j]=max(f[i-1][j],0.5*(a[i]+f[i-1][j-1]+f[i-1][j]));
		if(i<=m)f[i][i]=0.5*sum;
	}
	printf("%.6lf",1.0*sum-f[n][m]);
}