#include<iostream>

#include<cstdio>

#include<cmath>

#include<cstdlib>

#include<cstring>

#include<algorithm>

using namespace std;

int read()

{

    int x=,f=;char c=getchar();

    while (c<''||c>'') {if (c=='-') f=-;c=getchar();}

    while (c>=''&&c<='') x=(x<<)+(x<<)+(c^),c=getchar();

    return x*f;

}

#define N 21

#define T0 1E5

#define T1 1E-6

#define v 0.996

int n,m,a[N],sum[N],id[N],size[N];

double ave,ans=;

void anneal()

{

    for (int i=;i<=m;i++) id[i]=i,sum[i]=a[i],size[i]=;

    for (int i=m+;i<=n;i++) id[i]=rand()%m+,sum[id[i]]+=a[i],size[id[i]]++;

    double T=T0,tot=;

    for (int i=;i<=m;i++) tot+=(sum[i]-ave)*(sum[i]-ave);

    if (m<n)

    while (T>T1)

    {

        int x=rand()%n+;

        while (size[id[x]]==) x=rand()%n+;

        int y=rand()%m+;

        while (id[x]==y) y=rand()%m+;

        if (rand()<exp((-2.0*a[x]*(a[x]-sum[id[x]]+sum[y]))/T)*RAND_MAX)

        {

            tot+=*a[x]*(a[x]-sum[id[x]]+sum[y]);

            sum[y]+=a[x],sum[id[x]]-=a[x];

            size[y]++,size[id[x]]--;

            id[x]=y;

        }

        T*=v;

        ans=min(ans,tot);

    }

}

int main()

{

#ifndef ONLINE_JUDGE

    freopen("bzoj2428.in","r",stdin);

    freopen("bzoj2428.out","w",stdout);

    const char LL[]="%I64d\n";

#else

    const char LL[]="%lld\n";

#endif

    srand();

    n=read(),m=read();

    for (int i=;i<=n;i++) ave+=a[i]=read();

    ave/=m;

    for (int i=;i<=;i++) anneal();

    printf("%.2lf",sqrt(ans/m));

    return ;

}