#include<bits/stdc++.h>
#define int long long
#define fr(i,a,b) for(int i(a),end##i(b);i<=end##i;i++)
#define fd(i,a,b) for(int i(a),end##i(b);i>=end##i;i--)
#define de fprintf(stderr,"on-%d\n",__LINE__)
using namespace std;
const int maxn=2e5+5;
int a[maxn];
int n,m;
double f[maxn];
int s[maxn];

double S[maxn];
double w(int x,int y){
	return sqrt(1.*(s[y]-s[x])*(y-x));
}
struct node{
	int x,l,r;
};
deque<node> Q;
int d[maxn];
const double eps=1e-9;
int solve(double del){
	while(!Q.empty())Q.pop_back();
 	Q.push_back((node){0,1,n});
	fr(i,1,n){
		while(!Q.empty()&&Q.front().r<i)Q.pop_front();
		int p=Q.front().x;
		f[i]=f[p]+w(p,i)+del;
		//printf("%lld %lld %lld %lld %.4lf\n",i,p,Q.front().l,Q.front().r,f[i]);
		d[i]=d[p]+1;
		while(!Q.empty()&&f[Q.back().x]+w(Q.back().x,Q.back().l)>f[i]+w(i,Q.back().l)){
			Q.pop_back();
		}
		//printf("%lld\n",Q.back().x);
		if(Q.empty())Q.push_back((node){i,i+1,n});
		else if(f[Q.back().x]+w(Q.back().x,Q.back().r)>f[i]+w(i,Q.back().r)){
		//	printf("this\n");
			int l=Q.back().l,r=Q.back().r,pos=-1;
			//assert(r==n);
			while(l<=r){
				int mid=(l+r>>1);
				if(f[Q.back().x]+w(Q.back().x,mid)-eps<f[i]+w(i,mid)){
					pos=mid;
					l=mid+1;
				}
				else r=mid-1;
			}
			assert(pos!=-1);
			//printf("insert(%lld,%lld)\n",i,pos+1);
			Q.back().r=pos;
			Q.push_back((node){i,pos+1,n});
		//	printf("=%lld\n",Q.back().x);
		}else if(Q.back().r!=n)Q.push_back((node){i,Q.back().r+1,n}); 

	}
	//printf("%lf %lf\n",w(0,2)+w(2,7),w(0,3)+w(3,7));
	return d[n]; 
}
signed main(){
#ifdef pig
	freopen("pig.in","r",stdin);
	freopen("pig.out","w",stdout);
#endif
	cin>>n>>m;
	fr(i,1,n)scanf("%lld",&a[i]);
	fr(i,1,n)s[i]=s[i-1]+a[i];
	sort(a+1,a+n+1);
	double l=0,r=1e8,ans1,ans2,X,Y;
	while(r-l>eps){
		double mid=(l+r)/2;
		int t=solve(mid);
		if(t<=m){
			ans1=f[n]-mid*t;
			X=t;
		}
		if(t>m){
			ans2=f[n]-mid*t;
			Y=t;
		}
		if(t<m)r=mid;
		else l=mid;
	}
	printf("%.12lf\n",ans1+(ans2-ans1)/(Y-X)*(m-X));
	return 0;
}