#include<bits/stdc++.h>
#define db double
using namespace std;

const int N=2510;

int n,m,a[N],suf[N];
db f[N][N];

int main()
{
	scanf("%d%d",&n,&m);
	for(int i=1; i<=n; i++)
		scanf("%d",&a[i]);

	sort(a+1,a+1+n);

	f[0][0]=0;
	for(int i=1; i<=n; i++)
	{
		int cur=n-i+1;
		f[i][0]=(db)a[cur]+f[i-1][0];
		f[i][i]=(db)a[cur]/2.000000+f[i-1][i-1];
		for(int j=1; j<=min(i-1,m); j++)
			f[i][j]=(f[i-1][j-1]+f[i-1][j]+(db)a[cur])/2.000000;

	}
	// cout<<f[2][2]<<endl;
	db ans=1e9;
	for(int i=0; i<=m; i++)
		/*printf("%.3lf ",f[n][i]),*/ans=min(ans,f[n][i]);
	printf("%.6lf\n",f[n][m]);

	return 0;
}

/*
4 3
4 3 2 1
*/