#line 1 "F.cpp"
// #pragma GCC target("avx2,avx512f,avx512vl,avx512bw,avx512dq,avx512cd,avx512vbmi,avx512vbmi2,avx512vpopcntdq,avx512bitalg,bmi,bmi2,lzcnt,popcnt")
// #pragma GCC optimize("Ofast")

#line 2 "/home/sigma/comp/library/template.hpp"

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using uint = unsigned int;
using ull = unsigned long long;
#define rep(i,n) for(int i=0;i<int(n);i++)
#define rep1(i,n) for(int i=1;i<=int(n);i++)
#define per(i,n) for(int i=int(n)-1;i>=0;i--)
#define per1(i,n) for(int i=int(n);i>0;i--)
#define all(c) c.begin(),c.end()
#define si(x) int(x.size())
#define pb push_back
#define eb emplace_back
#define fs first
#define sc second
template<class T> using V = vector<T>;
template<class T> using VV = vector<vector<T>>;
template<class T,class U> bool chmax(T& x, U y){
	if(x<y){ x=y; return true; }
	return false;
}
template<class T,class U> bool chmin(T& x, U y){
	if(y<x){ x=y; return true; }
	return false;
}
template<class T> void mkuni(V<T>& v){sort(all(v));v.erase(unique(all(v)),v.end());}
template<class T> int lwb(const V<T>& v, const T& a){return lower_bound(all(v),a) - v.begin();}
template<class T>
V<T> Vec(size_t a) {
    return V<T>(a);
}
template<class T, class... Ts>
auto Vec(size_t a, Ts... ts) {
  return V<decltype(Vec<T>(ts...))>(a, Vec<T>(ts...));
}
template<class S,class T> ostream& operator<<(ostream& o,const pair<S,T> &p){
	return o<<"("<<p.fs<<","<<p.sc<<")";
}
template<class T> ostream& operator<<(ostream& o,const vector<T> &vc){
	o<<"{";
	for(const T& v:vc) o<<v<<",";
	o<<"}";
	return o;
}
constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n-1); }

#ifdef LOCAL
#define show(x) cerr << "LINE" << __LINE__ << " : " << #x << " = " << (x) << endl
void dmpr(ostream& os){os<<endl;}
template<class T,class... Args>
void dmpr(ostream&os,const T&t,const Args&... args){
	os<<t<<" ~ ";
	dmpr(os,args...);
}
#define shows(...) cerr << "LINE" << __LINE__ << " : ";dmpr(cerr,##__VA_ARGS__)
#define dump(x) cerr << "LINE" << __LINE__ << " : " << #x << " = {";  \
	for(auto v: x) cerr << v << ","; cerr << "}" << endl;
#else
#define show(x) void(0)
#define dump(x) void(0)
#define shows(...) void(0)
#endif

template<class D> D divFloor(D a, D b){
	return a / b - (((a ^ b) < 0 && a % b != 0) ? 1 : 0);
}
template<class D> D divCeil(D a, D b) {
	return a / b + (((a ^ b) > 0 && a % b != 0) ? 1 : 0);
}
#line 5 "F.cpp"

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);		//DON'T USE scanf/printf/puts !!
	cout << fixed << setprecision(20);

	int N,M; cin >> N >> M;
	V<ll> A(N); rep(i,N) cin >> A[i];
	sort(all(A));
	V<ll> S(N+1); rep(i,N) S[i+1] = S[i] + A[i];

	using P = pair<double,int>;
	double inf = 1e100;
	P infp(inf,-1);


	auto f_brute = [&](double penalty) -> pair<double,int> {
		V<P> dp(N+1, infp); dp[0] = P(0,0);
		rep1(i,N){
			rep(j,i){
				double cost = sqrt((S[i]-S[j]) * (i-j)) + penalty;
				chmin(dp[i],P(dp[j].fs+cost,dp[j].sc+1));
			}
		}
		return dp[N];
	};
	auto f = [&](double penalty) -> pair<double,int> {
		V<P> dp(N+1, infp); dp[0] = P(0,0);
		int pj = 0;	// [pj,i-1] をためす気持ち
		auto getcost = [&](int j, int i) -> double {
			return sqrt((S[i]-S[j]) * (i-j)) + penalty;
		};
		rep1(i,N){
			int npj = pj;
			for(int j=pj;j<i;j++){
				double cost = getcost(j,i);
				if(chmin(dp[i],P(dp[j].fs+cost,dp[j].sc+1))){
					npj = j;
				}else{
					break;
				}
			}
			for(int j=pj;j<min(pj+10,i);j++){
				double cost = getcost(j,i);
				chmin(dp[i],P(dp[j].fs+cost,dp[j].sc+1));
			}
			pj = npj-2; chmax(pj,0);
		}
		return dp[N];
	};

	double lb = 0, ub = 1e18;
	rep(_,90){
		double m = (lb+ub)/2;
		auto p = f(m);
		if(p.sc <= M) ub = m;
		else lb = m;
	}
	auto p = f(ub);
	show(ub);
	show(p);
	cout << p.fs - ub*M << endl;
}