#include <bits/stdc++.h>
using namespace std;
using ll =long long;
#define double long double
void solve(){
    int n,m;
    cin>>n>>m;
    vector<int> a(n+1);
    vector<ll> prea(n+1);
    for(int i=1;i<=n;i++)   cin>>a[i];
    sort(a.begin()+1,a.end());
    for(int i=1;i<=n;i++)   prea[i]=prea[i-1]+a[i];
    auto DP_2_deque=[&](double dj,int t)->pair<int,double> {
        vector<pair<int,double>> dp(n+1);
        struct node{int x,l,r;};
        deque<node> qe;
        auto get_w=[&](int l,int r)->double {
            return sqrt(1ll*(r-l)*(prea[r]-prea[l]))-dj;
        };
        auto get_l_r=[&](int l,int r)->pair<int,double> {
            return {dp[l].first+1,dp[l].second+get_w(l,r)};
        };
        auto cmp=[&](pair<int,double> x,pair<int,double> y)->bool {
            if(t==-1)   {
                if(x.second!=y.second)  return x.second<y.second;
                else return x.first<y.first;
            }
            else{
                if(x.second!=y.second)  return x.second>y.second;
                else return x.first<y.first;
            }
        };
        qe.push_back({0,1,n});
        for(int i=1;i<=n;i++){
            while(!qe.empty()&&qe.front().r<i)  qe.pop_front();
            dp[i]=get_l_r(qe.front().x,i);
            qe.begin()->l=i+1;
            while(!qe.empty()){
                int it=qe.back().l;
                if(!cmp(get_l_r(qe.back().x,it),get_l_r(i,it))) qe.pop_back();
                else break;
            }
            if(qe.empty())  qe.push_back({i,i+1,n});
            else{
                auto [x,l,r]=qe.back();
                int le=l,re=r;
                while(le<re){
                    int mid=(le+re+1)>>1;
                    if(cmp(get_l_r(x,mid),get_l_r(i,mid)))  le=mid;
                    else re=mid-1;
                }
                qe.rbegin()->r=le;
                if(le+1<=n) qe.push_back({i,le+1,n});
            }
        }
        return dp[n];
    };
    double maxn=0;
    for(int i=1;i<=n;i++)   maxn+=n*1.0/a[i];
    double le=0,re=maxn;
    const double eps=1e-15;
    while(re>le+eps){
        double mid=(le+re)/2;
        pair<int,double> res=DP_2_deque(-mid,-1);
        if(res.first<=m)    re=mid;
        else le=mid;
    }
    pair<int,double> res=DP_2_deque(-le,-1);
    cout<<setprecision(14)<<res.second-m*le<<'\n';
}
int main() {
    ios::sync_with_stdio(0),cin.tie(0);
    int _ =1;
    while(_--)  solve();
    return 0;
}