#include<iostream>
#include<iomanip>
#include<cstdio>
#include<cstdlib>
#include<cassert>

#include<cmath>
#include<functional>
#include<algorithm>

#include<vector>
#include<string>
#include<queue>
#include<stack>
#include<set>
#include<unordered_set>
#include<map>
#include<unordered_map>
#include<bitset>
#include<tuple>

#define TEST {IS_TEST=true;}
#define fi first
#define se second
#define pb(x) {push_back(x);}

using namespace std;
using ll = int_fast64_t;
using v_b = vector <bool>;
using v_ll = vector <ll>;
using str = string;
using v_str = vector <string>;
using p_ll = pair < ll,ll >;
using vv_b = vector < v_b >;
using vv_ll = vector < v_ll >;
using vp_ll = vector < p_ll >;
using vvv_ll = vector < vv_ll >;
using vvp_ll = vector < vp_ll >;

using ld = long double;
using v_ld = vector <ld>;
using vv_ld = vector<v_ld>;

bool IS_TEST=false;
ll ll_min64=1LL<<63;
ll ll_max64=~ll_min64;
ll ll_min32=1LL<<31;
ll ll_max32=~ll_min32;

ll MOD = 1000000007;

/*displaying functions for debug*/
template<class T> void show2(const T &x){cout << x;}
template<class T1,class T2> void show2(const pair<T1,T2> &x){
  cout << "{" << show2(x.first) << "," << show2(x.second) << "}";
}
template<class T> void show(const T &x){
  if (!IS_TEST) return;
  show2(x); cout <<endl;
}
template<class T> void v_show(const T &v, ll n=-1){
  if (!IS_TEST) return;
  auto itr=v.begin(); ll m=n;
  while(itr!=v.end() && m!=0 ){ show2(*itr); cout << " "; itr++; m--;}
  cout << endl;
}
template<class T> void vv_show(const T &v, ll n=-1){
  if (!IS_TEST) return;
  cout << "--------------------------------\n";
  auto itr=v.begin(); ll m=n;
  while(itr!=v.end() && m!=0 ){ v_show(*itr,n); itr++; m--;}
  cout << "--------------------------------" << endl;
}
/*--------------------------------*/

/*loading integers*/
void load(ll& x1){ cin >> x1; }
void load(ll& x1,ll& x2){ cin >> x1 >> x2; }
void load(ll& x1,ll& x2,ll& x3){ cin >> x1 >> x2 >> x3; }
void load(ll& x1,ll& x2,ll& x3,ll& x4){ cin >> x1 >> x2 >> x3 >> x4; }
void v_load(ll n, v_ll& v1, ll head=0, ll tail=0, ll init=0){
    ll m=n+head+tail; v1.assign(m,init);
    for (ll i=0;i<n;i++){ cin >> v1[i+head]; }
}
void v_load(ll n, v_ll& v1 ,v_ll& v2, ll head=0, ll tail=0, ll init=0){
    ll m=n+head+tail; v1.assign(m,init); v2.assign(m,init);
    for (ll i=0;i<n;i++){ cin >> v1[i+head] >> v2[i+head]; }
}
void v_load(ll n, v_ll& v1 ,v_ll& v2, v_ll& v3, ll head=0, ll tail=0, ll init=0){
    ll m=n+head+tail; v1.assign(m,init); v2.assign(m,init); v3.assign(m,init);
    for (ll i=0;i<n;i++){ cin >> v1[i+head] >> v2[i+head] >> v3[i+head]; }
}
void v_load(ll n, v_ll& v1 ,v_ll& v2, v_ll& v3, v_ll& v4, ll head=0, ll tail=0, ll init=0){
    ll m=n+head+tail; v1.assign(m,init); v2.assign(m,init); v3.assign(m,init); v4.assign(m,init);
    for (ll i=0;i<n;i++){ cin >> v1[i+head] >> v2[i+head] >> v3[i+head] >> v4[i+head]; }
}
/*--------------------------------*/

v_ll ll_mg(ll x1=ll_max64, ll x2=ll_max64, ll x3=ll_max64, ll x4=ll_max64){
    v_ll x{x1,x2,x3,x4};
    sort(x.begin(),x.end());
    return x;
}

template<class T> void ch_max(T& x,const T& y){ if (x>=y) return; x=y; }
template<class T> void ch_min(T& x,const T& y){ if (x<=y) return; x=y; }
template<class T,class S> void ch_max(T& x,S& xx,const T& y,const S& yy){
  if (x>=y) return; x=y; xx=yy;
}
template<class T,class S> void ch_min(T& x,S& xx,const T& y,const S& yy){
  if (x<=y) return; x=y; xx=yy;
}

template<class T> void quit(T x){cout << x << endl; exit(0);}

ll rem(ll x,ll y){ ll z=x%y; return z>=0?z:z+y; }

ll N,M,ans;
v_ll A,B,C,D;


v_ll fft(const v_ll& x, bool inv=false, ll n_depth=-1, const ll mod=998244353, const ll rt=31){
    if (n_depth<0) {n_depth=0; while (1LL<<++n_depth < x.size());}
    ll n=1LL<<n_depth;
    v_ll arr(n,0);
    v_ll temp(n,0);
    v_ll w(24,rt);
    for (ll i=22;i>=0;i--){ w[i]=w[i+1]*w[i+1]%mod; }
    for (ll i=0;i<x.size();i++){ arr[i]=x[i]; }
    ll depth,cell,n_cell;
    ll div=1;
    for (depth=1,cell=n,n_cell=1;cell>1;depth++,cell>>=1,n_cell<<=1){
        ll mul=1;
        if (inv) {div*=(mod+1)>>1; div%=mod;}
        for (ll i=0;i<n;i++){ temp[i]=arr[i]; }
        for (ll i=0;i<n_cell;i++){
            for (ll j=0;j<cell/2;j++){
                arr[i*cell/2+j]=(temp[i*cell+j]+mul*temp[i*cell+j+cell/2])%mod;
                arr[i*cell/2+j+n/2]=(temp[i*cell+j]+(mod-mul)*temp[i*cell+j+cell/2])%mod;
            }
            mul=mul*w[depth]%mod;
        }
    }
    if (inv) for (ll i=0;i<n;i++){ arr[i]*=div; arr[i]%=mod; }
    return arr;
}
v_ll convolute(const v_ll& x,const v_ll& y){
    ll n_depth=0;
    while (1LL<<++n_depth < max(x.size(),y.size()));
    n_depth++;
    ll mod,rt,rti;
    mod = 998244353; rt=31; rti=128805723;
    v_ll xx=fft(x,false,n_depth,mod,rt);
    v_ll yy=fft(y,false,n_depth,mod,rt);
    v_ll zz(xx.size(),0);
    for (ll i=0;i<xx.size();i++) { zz[i]=(xx[i]*yy[i])%mod; }
    v_ll z=fft(zz,true,n_depth,mod,rti);
    return z;
}
ll max(ll x,ll y){ return x>y?x:y;}
ll min(ll x,ll y){ return x<y?x:y;}

int main(){
    load(N,M);
    v_load(N,A);
    B.assign(100001,0);
    C.assign(100001,0);
    D.assign(100001,0);
    for (ll i=0;i<N;i++){
      B[A[i]]++;
    }
    for (ll i=0;i<100001;i++){
      C[i]=B[i]/500;
      D[i]=B[i]%500;
    }
    //TEST;
    v_show(B,100);
    v_show(C,100);
    v_show(D,100);
    
    v_ll a=convolute(C,C);
    v_ll b=convolute(C,D);
    v_ll c=convolute(D,D);
    ll remain=M;
    ans=0;
    for (ll i=200000;i>=0;i--){
      ll x=a[i]*500*500+b[i]*500*2+c[i];
      ans+=i*min(remain,x);
      remain-=x;
      if (remain<=0)break;
    }
    cout << ans << endl;
}




Measured memory_bytes: 17784