#include<bits/stdc++.h>
using namespace std;
using i64=long long;
const int N=2e5+5;
const long double eps=1e-11;
int n,m,s[N],g[N],top;
long double sum[N],f[N];
struct Node{int l,r,x;}base[N];
inline long double w(int l,int r){
	return sqrt((sum[r]-sum[l])*(r-l));
}
inline long double turn(int x,int y){
	return f[x]+w(x,y);
}
inline bool le(long double x,long double y){
	return x<=y||fabs(x-y)<=eps;
}
inline void solve(long double mid){
	Node *q=base;
	top=1,q[1]=(Node){1,n,0};
	for(int i=1;i<=n;i++){
		f[i]=turn(q[1].x,i)-mid,g[i]=g[q[1].x]+1;
		if(q[1].r==i)	q=q+1,--top;
		else	++q[1].l;
		while(top&&le(turn(i,q[top].l),turn(q[top].x,q[top].l)))	--top;
		if(top&&le(turn(i,q[top].r),turn(q[top].x,q[top].r))){
			int l=q[top].l,r=q[top].r;
			while(l<r){
				int mid=(l+r)/2;
				if(le(turn(i,mid),turn(q[top].x,mid)))	r=mid;
				else	l=mid+1;
			}
			q[top].r=l;
		}
		if(top&&q[top].r<n)	++top,q[top]=(Node){q[top-1].r+1,n,i};
		else if(!top)	q[++top]=(Node){i+1,n,i};
	}
}
int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;i++)	scanf("%d",&s[i]);
	sort(s+1,s+n+1);
	for(int i=1;i<=n;i++)	sum[i]=sum[i-1]+s[i];
	long double l=-20000000000.0,r=0,ans=-1;
	while(r-l>=eps){
		long double mid=(l+r)/2;
		solve(mid);
		if(g[n]<m)	l=mid;
		else if(g[n]>m)	r=mid;
		else{ans=f[n]+mid*g[n];break;}
	}
	printf("%.9Lf\n",ans);
	return 0;
}