#include<bits/stdc++.h>

#define ll long long
#define ull unsigned long long
#define fi first
#define se second
#define mkp make_pair
#define pb push_back
#define gc getchar
#define pc putchar
#define pii pair<int,int>
#define pil pair<int,long long>
#define pli pair<long long,int>
#define pll pair<long long,long long>
#define For(i,l,r) for(int i=(l);i<=(r);i++)
#define IFor(i,r,l) for(int i=(r);i>=(l);i--)
#define for_son(u) for(int e=head[u];e;e=nxt[e])
#define eps (1e-8)
#define INF 0x3f3f3f3f

using namespace std;

char aa;

namespace ljh
{

namespace IO
{
//    const int SIZ=1<<20;
//    char ibuf[SIZ],*p1,*p2,obuf[SIZ],*p3=obuf;
//  #define gc() (p1==p2&&(p2=(p1=ibuf)+fread(ibuf,1,SIZ,stdin),p1==p2)?EOF:*p1++)
//  void pc(const char &c)
//  {
//      if(p3-obuf==SIZ) fwrite(obuf,1,SIZ,stdout),p3=obuf;
//      *p3++=c;
//  }
    int rd()
    {
        int x=0,f=1;
        char ch=gc();
        while(!isdigit(ch)){
            if(ch=='-') f=-1;
            ch=gc();
        }
        while(isdigit(ch)){
            x=x*10+ch-'0';
            ch=gc();
        }
        return x*f;
    }
    void wr(int x,char ch)
    {
        if(x<0) x=-x,pc('-');
        static char st[12];
        int top=0;
        do{
            st[top++]=x%10+'0',x/=10;
        }while(x);
        while(top) pc(st[--top]);
        pc(ch);
    }
    ll ll_rd()
    {
        ll x=0;
        int f=1;
        char ch=gc();
        while(!isdigit(ch)){
            if(ch=='-') f=-1;
            ch=gc();
        }
        while(isdigit(ch)){
            x=x*10+ch-'0';
            ch=gc();
        }
        return x*f;
    }
    void ll_wr(ll x,char ch)
    {
        if(x<0) x=-x,pc('-');
        static char st[22];
        int top=0;
        do{
            st[top++]=x%10+'0',x/=10;
        }while(x);
        while(top) pc(st[--top]);
        pc(ch);
    }
}

using namespace IO;

const int N=2505,mod=0;

namespace tem
{
//  void add_edge(int u,int v){nxt[++cnt]=head[u],to[cnt]=v,head[u]=cnt;}
    int qpow(int a,int b,int p)
    {
        a%=p;
        int res=1;
        while(b){
            if(b&1) res=1ll*res*a%p;
            a=1ll*a*a%p;
            b>>=1;
        }
        return res;
    }
    int get_inv(int x){return qpow(x,mod-2,mod);}
    int lowbit(int x){return x&(-x);}
    void add(int &x,const int y){ x=(x+y>=mod?x+y-mod:x+y); }
//  int get(int x) {return fa[x]==x?x:fa[x]=get(fa[x])};
    int gcd(int a,int b){return b?gcd(b,a%b):a;}
    int lcm(int a,int b){return a/gcd(a,b)*b;}
}

using namespace tem;

int n,m;
double a[N],f[N][N];

void main()
{
	n=rd(),m=rd();
	For(i,1,n) a[i]=rd();
	sort(a+1,a+n+1);
	IFor(i,n,1){
		For(j,0,m){
			if(!j) f[i][j]=f[i+1][j]+a[i];
			else{
				f[i][j]=(a[i]+f[i+1][j-1]-f[i+1][j])*0.5+f[i+1][j];
			}
		}
	}
	printf("%.6lf\n",f[1][m]);
//  fwrite(obuf,1,p3-obuf,stdout);
}

/*
*/

}

char bb;

int main()
{
//  freopen("1.in","r",stdin);
//  freopen("1.out","w",stdout);    
//  int st=clock();
    ljh::main();
//  int ed=clock();
//  cerr<<ed-st<<"ms"<<endl;
//      cerr<<(&bb-&aa)/1024/1024<<"MB"<<endl;
//  fclose(stdin);
//  fclose(stdout);
    return 0;
}