#include<cstdio>

 #include<iostream>

 #include<cstdlib>

 #include<ctime>

 #include<cstring>

 #include<cmath>

 #include<algorithm>

 #define M 100

 using namespace std;

 int n,m,a[M],pos[M];

 double rev,sum[M],ans,mn=1e30,pre;

 void solve()

 {

     memset(sum,,sizeof(sum));

     ans=;

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

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

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

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

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

       ans+=(sum[i]-rev)*(sum[i]-rev);

     double T=;

     for(;T>0.1;)

       {

         int t1=rand()%n+,x=pos[t1],y=rand()%m+;

         if(x==y)

           continue;

         T*=0.9;

         pre=ans;

         ans-=(sum[x]-rev)*(sum[x]-rev);

         ans-=(sum[y]-rev)*(sum[y]-rev);

         sum[x]-=a[t1];

         sum[y]+=a[t1];

         ans+=(sum[x]-rev)*(sum[x]-rev);

         ans+=(sum[y]-rev)*(sum[y]-rev);

         if(ans>pre)

           {

             ans=pre;

             sum[x]+=a[t1];

             sum[y]-=a[t1];

           }

         else

           pos[t1]=y;

       }

     if(ans<mn)

       mn=ans;

     return;

 }

 int main()

 {

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

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

     {

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

     rev+=a[i];

     swap(a[i],a[rand()%i+]);

     }

     rev/=m;

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

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

     return ;

 }