#include <iostream>
#include <cmath>
#include <algorithm>
#include <iomanip>
using namespace std;
int v[200005];
//int opt[200005];
long long spar[200005];
pair<long double,int> dp[200005];
int n;
pair<long double ,int>best;
long double calc(int i ,int j)
{
    if(j==i)
    {
        return 1e9;
    }
    long double val=dp[i].first+sqrtl((spar[j]-spar[i])*(j-i));
    if(val<=best.first)
    {
        best.first=val;
        best.second=dp[i].second;
    }
    return val;
}
int check(long double plm)
{
    long double valus;
    int st,dr,mij;
    dp[0]={0,0};
    int opt=0;
    for(int i=1;i<=n;i++)
    {
        best={1e9,0};
        st=0;
        dr=i-1;
        while(st<=dr)
        {
            mij=(st+dr)/2;
            if(calc(mij,i)>=calc(mij+1,i))
            {
                st=mij+1;
            }
            else
            {
                dr=mij-1;
            }
        }
        dp[i].first=best.first;
        dp[i].second=best.second+1;
        dp[i].first+=plm;
    }
    return dp[n].second;
}
int main()
{
    int i,j,k,l,m,val;
    long double st=-1e9,dr=1e9,mij;
    cin>>n>>m;
    for(i=1;i<=n;i++)
    {
        cin>>v[i];
    }
    sort(v+1,v+n+1);
    for(i=1;i<=n;i++)
    {
        spar[i]=spar[i-1]+v[i];
    }
    for(i=1;i<=80;i++)
    {
        mij=(st+dr)/2;
        val=check(mij);
        if(val==m)
        {
            break;
        }
        else if(val>m)
        {
            st=mij;
        }
        else
        {
            dr=mij;
        }
    }
    cout<<fixed<<setprecision(12)<<dp[n].first-m*mij;
    return 0;
}