Language: C++

#include<bits/stdc++.h>
//ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<pii,int> ppii;
typedef pair<int,pii> pipi;
typedef pair<ll,ll> pll;
typedef pair<pll,ll> ppll;
typedef pair<ll,pll> plpl;
typedef tuple<ll,ll,ll> tl;
ll mod=1000000007;
ll mod2=998244353;
ll mod3=1000003;
ll mod4=998244853;
ll inf=1000000000000000000;
long double pi=acos(-1.0);
#define rep(i,m,n) for(ll i=m;i<n;i++)
#define rrep(i,n,m) for(ll i=n;i>=m;i--)
int dh[4]={1,-1,0,0};
int dw[4]={0,0,1,-1};
int ddh[8]={-1,-1,-1,0,0,1,1,1};
int ddw[8]={-1,0,1,-1,1,-1,0,1};
ll lmax(ll a,ll b){
    if(a<b)return b;
    else return a;
}
ll lmin(ll a,ll b){
    if(a<b)return a;
    else return b;
}
ll gcd(ll a,ll b){
    if(a<b)swap(a,b);
    if(b==0)return a;
    if(a%b==0)return b;
    return gcd(b,a%b);
}
ll Pow(ll n,ll k){
    ll ret=1;
    ll now=n;
    while(k>0){
        if(k&1)ret*=now;
        now*=now;
        k/=2;
    }
    return ret;
}
ll beki(ll n,ll k,ll md){
  ll ret=1;
  ll now=n;
  while(k>0){
    if(k%2==1){
      ret*=now;
      ret%=md;
    }
    now*=now;
    now%=md;
    k/=2;
  }
  return ret;
}
ll gyaku(ll n,ll md){
  return beki(n,md-2,md);
}
vector<complex<long double>> fft(vector<complex<long double>> a,int bo){
    int n=a.size();
    int h=0;
    while((1<<h)<n)h++;
    rep(i,0,n){
        ll j=0;
        rep(k,0,h)j|=((i>>k)&1)<<(h-1-k);
        if(i<j)swap(a[i],a[j]);
    }
    for(ll i=1;i<n;i*=2){
        rep(j,0,i){
            complex<long double> w=polar(1.0,(-(2*pi)/(2*i))*j*bo);
            for(ll k=0;k<n;k+=i*2){
                complex<long double> s=a[j+k];
                complex<long double> t=a[j+k+i]*w;
                a[j+k]=s+t;
                a[j+k+i]=s-t;
            }
        }
    }
    if(bo==-1)rep(i,0,n)a[i]/=n;
    return a;
}
vector<complex<long double>> fft(vector<long double> a,int bo){
    vector<complex<long double>> b(a.size());
    rep(i,0,a.size())b[i]=complex<long double>(a[i],0);
    return fft(b,bo);
}
vector<long double> convolve(vector<long double> a,vector<long double> b){
    int s=a.size()+b.size()-1;
    int t=1;
    while(t<s)t*=2;
    a.resize(t);
    b.resize(t);
    vector<complex<long double>> A=fft(a,1);
    vector<complex<long double>> B=fft(b,1);
    rep(i,0,t)A[i]*=B[i];
    A=fft(A,-1);
    rep(i,0,s)a[i]=A[i].real();
    return a;
}
int main(){
    ios::sync_with_stdio(false);cin.tie(0);
    ll n,m;cin>>n>>m;
    vector<long double> a(100001),b(100001);
    ll cn[100001];
    fill(cn,cn+100001,0);
    rep(i,0,n){
        int t;cin>>t;
        cn[t]++;
       
    }
    rep(i,0,100001){
        a[i]=(long double)cn[i];
        b[i]=(long double)cn[i];
    }
    vector<long double> anss=convolve(a,b);
    ll ans[anss.size()];
    rep(i,0,anss.size())ans[i]=(ll)(anss[i]+0.5);
    ll sum=0;
    ll cnt=0;
    //cout<<anss.size()<<endl;
    rrep(i,anss.size()-1,0){
        if(cnt+ans[i]>m){
            sum+=i*(m-cnt);
            break;
        }
        else{
            sum+=i*ans[i];
            cnt+=ans[i];
        }
    }
    cout<<sum<<endl;
}


Compiler output:
a.cc: In function 'std::vector<std::complex<long double> > fft(std::vector<std::complex<long double> >, int)':
a.cc:77:41: error: no matching function for call to 'polar(double, long double)'
   77 |             complex<long double> w=polar(1.0,(-(2*pi)/(2*i))*j*bo);
      |                                    ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127,
                 from a.cc:1:
/usr/include/c++/14/complex:967:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::polar(const _Tp&, const _Tp&)'
  967 |     polar(const _Tp& __rho, const _Tp& __theta)
      |     ^~~~~
/usr/include/c++/14/complex:967:5: note:   template argument deduction/substitution failed:
a.cc:77:41: note:   deduced conflicting types for parameter 'const _Tp' ('double' and 'long double')
   77 |             complex<long double> w=polar(1.0,(-(2*pi)/(2*i))*j*bo);
      |                                    ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~