/**************************************************************

     Problem: 2428

     User: 137shoebills

     Language: C++

     Result: Accepted

     Time:4884 ms

     Memory:1304 kb

 ****************************************************************/

 #include<iostream>

 #include<cstdio>

 #include<cstring>

 #include<algorithm>

 #include<cmath>

 using namespace std;

 const int maxn=;

 int n,m;

 int a[maxn]={};

 int org[maxn]={};

 int sum[maxn]={};

 double xa=,ans=;

 void doit(){

     memset(sum,,sizeof(sum));

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

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

         sum[org[i]]+=a[i];

     }

     double cnt=,lcnt,T=;

     for(int j=;j<=m;j++){

         cnt+=((double)sum[j]-xa)*((double)sum[j]-xa);

     }cnt/=(double)m;

     while(T>0.1){//温度设定

         T*=0.9;

         int x=rand()%n+,y,mi=;//随机选一个数,最小和对应位置，最小和

         if(T>)//温度高贪心温度低随机

             for(int i=;i<=m;i++){if(mi>sum[i]){mi=sum[i];y=i;}}

         else y=rand()%m+;

         if(y==org[x]){continue;}

         sum[org[x]]-=a[x];sum[y]+=a[x];

         lcnt=cnt;cnt=;//重新算方差

         for(int j=;j<=m;j++){

             cnt+=((double)sum[j]-xa)*((double)sum[j]-xa);

         }cnt/=(double)m;

         if(cnt<lcnt)org[x]=y;

         else if(rand()%>T)org[x]=y,cnt=lcnt;

         else{

             sum[org[x]]+=a[x];sum[y]-=a[x];

         }

         if(cnt<ans)ans=cnt;//更新答案

     }

 }

 int main(){

     //freopen("data.in","r",stdin);

     //freopen("data.out","w",stdout);

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

     //srand(20010123);

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

         scanf("%d",&a[i]);xa+=a[i];

     }xa/=(double)m;

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

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

     return ;

 }