#include<bits/stdc++.h>
using namespace std;
const int N=2e5+5;
using ld=long double;
const ld inf=1e18+7;
const ld eps=1e-10;
int n,m;ld s[N],f[N];
int p[N];ld calc(int x,int y)
{return f[x]+sqrtl((s[y]-s[x])*(y-x));}
int L[N],R[N],x[N],ql,qr;
int main()
{
    cin>>n>>m;for(int i=1;i<=n;i++)cin>>s[i];
    sort(s+1,s+n+1);for(int i=1;i<=n;i++)s[i]+=s[i-1];
    ld l=0,r=1e11,c;ld rs=0;
    for(int tm=1;tm<=100;tm++)
    {
        c=(l+r)*0.5;f[0]=0;
        ql=qr=1;L[1]=1,R[1]=n,x[1]=0;
        for(int i=1;i<=n;i++)
        {
            p[i]=x[ql],f[i]=calc(p[i],i)+c;
            if(i==R[ql])ql++;else L[ql]=i+1;
            while(qr>=ql)
            {
                int ps=L[qr];
                if(calc(x[qr],ps)-
                calc(i,ps)<eps)
                qr--;else break;
            }if(qr<ql)
            {
                qr++;
                L[qr]=i+1,R[qr]=n,x[qr]=i;
                continue;
            }int l=L[qr]+1,r=R[qr]+1,mid;
            int x=i,y=::x[qr];while(l<r)
            {
                mid=(l+r)>>1;
                if(calc(y,mid)-calc(x,mid)<eps)
                r=mid;else l=mid+1;
            }R[qr]=l-1;qr++;
            L[qr]=l,R[qr]=n,::x[qr]=i;
        }int ps=n,ct=0;
        while(ps)ct++,ps=p[ps];
        if(ct==m){rs=f[n]-c*m;break;}
        if(ct<m)r=c;else l=c;
    }cout<<setprecision(12)<<rs<<'\n';
    return 0;
}