Language: C++

#include <bits/stdc++.h>
using namespace std;
//#define int long long
//TEMPLATE START---------------8<---------------8<---------------8<---------------8<---------------//
typedef long long ll;       typedef long double ld;  typedef pair<int,int> pii; typedef pair<ll,ll> pll;  typedef vector<int> vi;   typedef vector<ll> vl;
typedef vector<string> vst; typedef vector<bool> vb; typedef vector<ld> vld;    typedef vector<pii> vpii; typedef vector<pll> vpll; typedef vector<vector<int> > vvi;
const int INF = (0x7FFFFFFFL); const ll INFF = (0x7FFFFFFFFFFFFFFFL); const string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const int MOD = 1e9 + 7;       const int MODD = 998244353;            const string alphabet = "abcdefghijklmnopqrstuvwxyz";
const double PI = acos(-1.0);  const double EPS = 1e-9;               const string Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
int dx[9] = { 1, 0, -1,  0,  1, -1, -1, 1, 0 };
int dy[9] = { 0, 1,  0, -1, -1, -1,  1, 1, 0 };
#define ln '\n'
#define scnaf scanf
#define sacnf scanf
#define sancf scanf
#define SS(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t){cin >> t;}template<typename First, typename...Rest> void MACRO_VAR_Scan(First& first, Rest&...rest){cin >> first;MACRO_VAR_Scan(rest...);}
#define SV(type,c,n) vector<type> c(n);for(auto& i:c)cin >> i;
#define SVV(type,c,n,m) vector<vector<type>> c(n,vector<type>(m));for(auto& r:c)for(auto& i:r)cin >> i;
template<class T,class U>ostream &operator<<(ostream &o,const pair<T,U>&j){o<<"{"<<j.first<<", "<<j.second<<"}";return o;}
template<class T,class U>ostream &operator<<(ostream &o,const map<T,U>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const set<T>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const multiset<T>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const vector<T>&j){o<<"{";for(int i=0;i<(int)j.size();++i)o<<(i>0?", ":"")<<j[i];o<<"}";return o;}
inline int print(void){cout << endl; return 0;}
template<class Head> int print(Head&& head){cout << head;print();return 0;} template<class Head,class... Tail> int print(Head&& head,Tail&&... tail){cout<<head<<" ";print(forward<Tail>(tail)...);return 0;}
#ifdef LOCAL
inline int dump(void){cerr << endl; return 0;}
template<class Head> int dump(Head&& head){cerr << head;dump();return 0;}
template<class Head,class... Tail> int dump(Head&& head,Tail&&... tail){cerr<<head<<" ";dump(forward<Tail>(tail)...);return 0;}
#define debug(...) do{cerr<<__LINE__<<":	"<<#__VA_ARGS__<<" = ";dump(__VA_ARGS__);}while(0)
#define ER(x)  cerr << #x << " = " << (x) << endl;
#define ERV(v) {cerr << #v << " : ";for(const auto& xxx : v){cerr << xxx << " ";}cerr << "\n";}
#else
#define dump(...)
#define debug(...)
#define ER(x)
#define ERV(v)
#endif
template<typename T> void PA(T &a){int ASIZE=sizeof(a)/sizeof(a[0]);for(int ii=0;ii<ASIZE;++ii){cout<<a[ii]<<" \n"[ii==ASIZE-1];}}
template<typename T> void PV(T &v){int VSIZE=v.size();for(int ii=0;ii<VSIZE;++ii){cout<<v[ii]<<" \n"[ii==VSIZE-1];}}
inline int YES(bool x){cout<<((x)?"YES":"NO")<<endl;return 0;} inline int Yes(bool x){cout<<((x)?"Yes":"No")<<endl;return 0;}  inline int yes(bool x){cout<<((x)?"yes":"no")<<endl;return 0;}
inline int yES(bool x){cout<<((x)?"yES":"nO")<<endl;return 0;} inline int Yay(bool x){cout<<((x)?"Yay!":":(")<<endl;return 0;}
template<typename A,typename B> void sankou(bool x,A a,B b){cout<<((x)?(a):(b))<<endl;}
#define _overload3(_1,_2,_3,name,...) name
#define _REP(i,n) REPI(i,0,n)
#define REPI(i,a,b) for(ll i=ll(a);i<ll(b);++i)
#define REP(...) _overload3(__VA_ARGS__,REPI,_REP,)(__VA_ARGS__)
#define _RREP(i,n) RREPI(i,n,0)
#define RREPI(i,a,b) for(ll i=ll(a);i>=ll(b);--i)
#define RREP(...) _overload3(__VA_ARGS__,RREPI,_RREP,)(__VA_ARGS__)
#define EACH(e,v) for(auto& e : v)
#define PERM(v) sort((v).begin(),(v).end());for(bool c##p=1;c##p;c##p=next_permutation((v).begin(),(v).end()))
#define ADD(a,b) a=(a+ll(b))%MOD
#define MUL(a,b) a=(a*ll(b))%MOD
inline ll MOP(ll x,ll n,ll m=MOD){ll r=1;while(n>0){if(n&1)(r*=x)%=m;(x*=x)%=m;n>>=1;}return r;}
inline ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}inline ll lcm(ll a,ll b){return a*b/gcd(a,b);}inline ll POW(ll a,ll b){ll c=1ll;do{if(b&1)c*=1ll*a;a*=1ll*a;}while(b>>=1);return c;}
template<typename T,typename A,typename B> inline bool between(T x,A a,B b) {return ((a<=x)&&(x<b));}template<class T> inline T sqr(T x){return x*x;}
template<typename A,typename B> inline bool chmax(A &a,const B &b){if(a<b){a=b;return 1;}return 0;}
template<typename A,typename B> inline bool chmin(A &a,const B &b){if(a>b){a=b;return 1;}return 0;}
#define tmax(x,y,z) max((x),max((y),(z)))
#define tmin(x,y,z) min((x),min((y),(z)))
#define PB emplace_back
#define MP make_pair
#define MT make_tuple
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define SORT(v) sort((v).begin(),(v).end())
#define RSORT(v) sort((v).rbegin(),(v).rend())
#define EXIST(s,e) (find((s).begin(),(s).end(),(e))!=(s).end())
#define EXISTST(s,c) (((s).find(c))!=string::npos)
#define POSL(x,val) (lower_bound(x.begin(),x.end(),val)-x.begin())
#define POSU(x,val) (upper_bound(x.begin(),x.end(),val)-x.begin())
#define GEQ(x,val) (int)(x).size() - POSL((x),(val))
#define GREATER(x,val) (int)(x).size() - POSU((x),(val))
#define LEQ(x,val) POSU((x),(val))
#define LESS(x,val) POSL((x),(val))
#define SZV(a) int((a).size())
#define SZA(a) sizeof(a)/sizeof(a[0])
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
#define MEMINF(a) memset(a,0x3f,sizeof(a))
#define FILL(a,b) memset(a,b,sizeof(a))
#define UNIQUE(v) sort((v).begin(),(v).end());(v).erase(unique((v).begin(),(v).end()),(v).end())
struct abracadabra{
    abracadabra(){
        cin.tie(0); ios::sync_with_stdio(0);
        cout << fixed << setprecision(20);
        cerr << fixed << setprecision(5);
    };
} ABRACADABRA;

//TEMPLATE END---------------8<---------------8<---------------8<---------------8<---------------//

namespace FastFourierTransform {
    using C = complex< double >;
    void DiscreteFourierTransform(vector< C > &F, bool rev) {
        const int N = (int)F.size();
        const double PI = (rev ? -1 : 1) * acos(-1);
        for (int i = 0, j = 1; j + 1 < N; ++j) {
            for (int k = N >> 1; k > (i ^= k); k >>= 1);
            if (i > j) swap(F[i], F[j]);
        }
        C w, s, t;
        for (int i = 1; i < N; i <<= 1) {
            for (int k = 0; k < i; ++k) {
                w = polar(1.0, PI / i * k);
                for (int j = 0; j < N; j += i * 2) {
                    s = F[j + k];
                    t = C(
                        F[j + k + i].real() * w.real() - F[j + k + i].imag() * w.imag(),
                        F[j + k + i].real() * w.imag() + F[j + k + i].imag() * w.real()
                    );
                    F[j + k] = s + t, F[j + k + i] = s - t;
                }
            }
        }
        if (rev) for (int i = 0; i < N; ++i) F[i] /= N;
    }
    vector<int_fast64_t> Multiply(const vector<int> &A, const vector<int> &B) {
        int sz = 1;
        while (sz < A.size() + B.size() - 1) sz <<= 1;
        vector< C > F(sz), G(sz);
        for (int i = 0; i < A.size(); ++i) F[i] = A[i];
        for (int i = 0; i < B.size(); ++i) G[i] = B[i];
        DiscreteFourierTransform(F, false);
        DiscreteFourierTransform(G, false);
        for (int i = 0; i < sz; ++i) F[i] *= G[i];
        DiscreteFourierTransform(F, true);
        vector<int_fast64_t> X(A.size() + B.size() - 1);
        for (int i = 0; i < A.size() + B.size() - 1; ++i) X[i] = F[i].real() + 0.5;
        return X;
    }
};

/*
・高速フーリエ変換
    > O(NlogN)
[備考] 畳み込み(c_i = Sum[j = 0..i] a_j * b_(i - j))を高速に行える.
[使用例] vector<int_fast64_t> C = FastFourierTransform::Multiply(A, B);      // AとBの畳み込みの結果をCに代入
[参考] https://www.creativ.xyz/fast-fourier-transform/
*/

signed main() {

    SS(ll, N, M);
    SV(ll, A, N);

    // vector<int> his(35, 0);
    vector<int> his(100100, 0);
    EACH(e, A) his[e]++;

    vector<int_fast64_t> C = FastFourierTransform::Multiply(his, his);

    ll res = 0;

    int sz = C.size();
    // REP(i, sz) debug(i, C[i]);
    RREP(i, sz - 1) {
        ll need = min(M, C[i]);
        res += need * (ll)i;
        M -= need;
        if (M <= 0) break;
    }

    print(res);

}

Compiler output:
a.cc: In function 'int main()':
a.cc:160:22: error: no matching function for call to 'min(ll&, __gnu_cxx::__alloc_traits<std::allocator<long int>, long int>::value_type&)'
  160 |         ll need = min(M, C[i]);
      |                   ~~~^~~~~~~~~
In file included from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:233:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::min(const _Tp&, const _Tp&)'
  233 |     min(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:233:5: note:   template argument deduction/substitution failed:
a.cc:160:22: note:   deduced conflicting types for parameter 'const _Tp' ('long long int' and '__gnu_cxx::__alloc_traits<std::allocator<long int>, long int>::value_type' {aka 'long int'})
  160 |         ll need = min(M, C[i]);
      |                   ~~~^~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::min(const _Tp&, const _Tp&, _Compare)'
  281 |     min(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: note:   candidate expects 3 arguments, 2 provided
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:5686:5: note: candidate: 'template<class _Tp> constexpr _Tp std::min(initializer_list<_Tp>)'
 5686 |     min(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5686:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5696:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::min(initializer_list<_Tp>, _Compare)'
 5696 |     min(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5696:5: note:   template argument deduction/substitution failed:
a.cc:160:22: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
  160 |         ll need = min(M, C[i]);
      |                   ~~~^~~~~~~~~