//Twenty

#include<cstdio>

#include<cstdlib>

#include<iostream>

#include<algorithm>

#include<cmath>

#include<cstring>

#include<queue>

#include<vector>

#include<ctime>

#define Ti 1e4

const int maxn=;

using namespace std;

int n,from[maxn],m;

double tp,ave,ans,p[maxn],sum[maxn];

double pf(double x) {return x*x;}

double solve() {

    double res=;

    for(int i=;i<=m;i++) sum[i]=;

    for(int i=;i<=n;i++) {

        from[i]=rand()%m+;

        sum[from[i]]+=p[i];

    }

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

        res+=pf(sum[i]-ave);

    double T=Ti;

    while(T>0.1) {

        int x=rand()%n+,y;

        y=T>?min_element(sum+,sum+m+)-sum:rand()%m+;

        tp=res-pf(sum[y]-ave)+pf(sum[y]+p[x]-ave)-pf(sum[from[x]]-ave)+pf(sum[from[x]]-p[x]-ave);

        if(tp<res||exp((res-tp)*Ti/T)>(double)rand()/RAND_MAX) {

            res=tp;

            sum[y]+=p[x];

            sum[from[x]]-=p[x];

            from[x]=y;

        }

        T*=0.9;

    }

    return res;

}

int main()

{

    srand();

    scanf("%d%d",&n,&m);

    for(int i=;i<=n;i++) {

        scanf("%lf",&p[i]);

        ave+=p[i];

    }

    ave/=m;

    ans=solve();

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

        ans=min(ans,solve());

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

    return ;

}