#include<cmath>
#include<cstdio>
#include<vector>
#include<cstring>
#include<iostream>
#include<algorithm>
#define fi first
#define se second
using namespace std;
typedef long long LL;
typedef long double LD;
typedef pair<int,int> PII;
const int N=2e5+9;
int n,m; LL s[N];
LD f[N]; int g[N];
int q[N],r[N],hh,tt;
LD get(int i,int j){
//	printf("g %d %d %Lf %Lf\n",i,j,sqrtl((i-j)*(s[i]-s[j])),f[j]+sqrtl((i-j)*(s[i]-s[j])));
	return f[j]+sqrtl((i-j)*(s[i]-s[j]));
}
int find(int x,int y){
	int l=x,r=n+1;
	while(l<r){
		int mid=l+r>>1;
		if(get(mid,x)>=get(mid,y)) r=mid;
		else l=mid+1;
	}
	return l;
}
int check(LD mid){
	q[hh=tt=0]=0;
	for(int i=1;i<=n;i++){
		while(hh<tt&&r[hh]<=i) hh++;
		f[i]=get(i,q[hh])+mid; g[i]=g[q[hh]]+1;
//		printf("%d %d %Lf %Lf %d\n",q[hh],i,sqrtl((i-q[hh])*(s[i]-s[q[hh]])),f[i],g[i]);
		while(hh<tt&&r[tt-1]>=find(q[tt],i)) tt--;
		r[tt]=find(q[tt],i); q[++tt]=i;
	}
	return g[n];
}
int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;i++) scanf("%lld",&s[i]);
	sort(s+1,s+1+n);
	for(int i=1;i<=n;i++) s[i]+=s[i-1];
	LD l=0,r=1e18/2e5;
	while(r-l>1e-9){
		LD mid=(l+r)/2;
		if(check(mid)<=m) r=mid;
		else l=mid;
	}
	check(r);
	printf("%.9Lf\n",f[n]-r*m);
    return 0;
}