#include<iostream>

#include<cstdio>

#include<cstring>

#include<cctype>

#include<cstdlib>

#include<cmath>

#include<ctime>

#define re register

using namespace std;

template <typename T> inline T min(T &a,T &b) {return a<b ?a:b;}

template <typename T> inline T max(T &a,T &b) {return a>b ?a:b;}

template <typename T> inline void read(T &x){

    char c=getchar(); x=; bool f=;

    while(!isdigit(c)) f= !f||c=='-' ? :,c=getchar();

    while(isdigit(c)) x=(x<<)+(x<<)+(c^),c=getchar();

    x= f ? x:-x;

}

template <typename T> inline T squ(T a) {return a*a;}

typedef double db;

int n,m,a[],sum[];

db f[][],ans=1e15,ave;

inline db calc(){ //dp:设f[i][j]表示当前为第i个位置，分成j块

    memset(f,,sizeof(f)); f[][]=;

    for(re int i=;i<=n;++i) sum[i]=sum[i-]+a[i];

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

        for(re int j=;j<=i;++j)

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

                f[i][j]=min(f[i][j],f[k][j-]+squ(sum[i]-sum[k]-ave));

    return f[n][m];

}

inline void smt(){

    for(db t=;t>1e-;t*=0.993){ //自行修改

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

        swap(a[x],a[y]); //随机交换2个数字

        db _val=calc(),delta=_val-ans;

        if(delta<) ans=_val;

        else if(exp(-delta/t)*RAND_MAX<=rand()) swap(a[x],a[y]); //一定概率接受非最优解

    }

}

int main(){

    srand(); srand(rand()); srand(rand());

    read(n),read(m);

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

    ave=ave/(db)m;

    while((db)clock()/CLOCKS_PER_SEC<0.825) smt(); //卡时。可以先看下跑一遍退火时间多少，再修改限制值（记得多留点时间给下面的代码）

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

    return ;

}