#include<cmath>

#include<queue>

#include<cstdio>

#include<cctype>

#include<cstdlib>

#include<cstring>

#include<iostream>

#include<algorithm>

using namespace std;

typedef long double dl;

const int N=;

const dl T=;

const dl eps=1e-;

const dl delta=0.998;

inline int read(){

    int X=,w=;char ch=;

    while(!isdigit(ch)){w|=ch=='-';ch=getchar();}

    while(isdigit(ch))X=(X<<)+(X<<)+(ch^),ch=getchar();

    return w?-X:X;

}

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

dl sum,rms,t,ans=9e18;

inline int sigma(int l,int r){

    return (s[r]-s[l-])*(s[r]-s[l-]);

}

inline int suan(){

    memset(f,,sizeof(f));

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

    s[i]=s[i-]+a[i];

    f[i][]=sigma(,i);

    for(int j=;j<=min(i,m);j++){

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

        f[i][j]=min(f[i][j],f[k][j-]+sigma(k+,i));

        }

    }

    }

    return f[n][m];

}

void simulate_anneal(){

    t=T;

    while(t>eps){

    int i=rand()%n+,j=rand()%n+;

    swap(a[i],a[j]);

    dl nans=suan();

    dl dans=nans-ans;

    if(nans<ans||rand()<exp(-dans/t)*RAND_MAX)ans=nans;

    else swap(a[i],a[j]);

    t*=delta;

    }

}

int main(){

    srand();

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

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

    simulate_anneal();

    rms=sum/m;

    ans=sqrt((ans-rms*rms*m)/m);

    printf("%.2Lf\n",ans);

    return ;

}