Language: C++

#include<bits/stdc++.h>
using namespace std;
 
#define IOS ios_base::sync_with_stdio(false);cin.tie(NULL); 
 
#define ff first
#define ss second
#define pb push_back
#define pf push_front
#define mp make_pair
#define in insert
#define ld long double
 
#define TRACE
#ifdef TRACE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
  cerr << name << " : " << arg1 << std::endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
  const char* comma = strchr(names + 1, ',');cerr.write(names, comma - names) << " : " << arg1<<" | ";__f(comma+1, args...);
}
#else
#define trace(...) 1
#endif
template<class T> ostream& operator<<(ostream &os, vector<T> V) {os << "[ "; for(auto v : V) os << v << " "; return os << "]";}
template<class L, class R> ostream& operator<<(ostream &os, pair<L,R> P) {return os << "(" << P.first << "," << P.second << ")";}
 
typedef long long int ll;
 
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<pii> vpii;
typedef vector<pll> vpll;
const ll mod =1e9+7;
const ll MAX_NODES =200001;
const ll MAX_LOG =50;
ll power(ll x, ll n) {
    ll res = 1;
    for (; n > 0; n >>= 1) {
    if (n & 1) res = (res * x) % mod;
    x = (x * x) % mod;
    }
    return res;
}

int main()
{
  #ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
  #endif
  IOS
  ll n;
  ll k;
  cin>>n>>k;
  ll a[n];
  vl pos,neg;
  ll z=0;
  for(ll i=0;i<n;i++)
  {
    cin>>a[i];
    if(a[i]>0)
      pos.pb(a[i]);
    else if(a[i]==0)
      z++;
      else
      {
        neg.pb(-a[i]);
      }
      
  }
  sort(pos.begin(),pos.end());
  sort(neg.begin(),neg.end());
  if(k<=(pos.size()*neg.size()))
  {
    ll l = -1e18,r = -1;
    ll fin=0;
    while(r>=l)
    {
      ll mid = (r+l)/2;
      ll temp = -mid;
      ll ans=0;
      for(ll i=0;i<neg.size();i++)
      {
        ll pp= ((temp%neg[i])?upper_bound(pos.begin(),pos.end(),temp/neg[i]):lower_bound(pos.begin(),pos.end(),temp/neg[i]))-pos.begin();
        ans+=(pos.size()-pp);
      }
      if(ans==k)
        fin=mid;
      if(ans>=k)
      {
        r= mid;
      }
      else
      {
        l=mid+1;
      }
    
    }
    cout<<l<<endl;
    return 0;

  }
  else
  {
    k-=pos.size()*neg.size();

  }
  if(k<=(z*(n-z)+(z*(z-1))/2))
    {
      cout<<0<<endl;
      return 0;
    }
  else
  { 
    k-=(z*(n-z)+(z*(z-1))/2);
    ll l = 1,r = 1e18;
    //trace(k);
    ll fin = 0;
    while(r>=l)
    {
      ll mid = (r+l)/2;
      ll temp = mid;
      ll ans=0;
      
      for(ll i=0;i<neg.size();i++)
      {
        ll pp= upper_bound(neg.begin(),neg.end(),temp/neg[i])-neg.begin();
        if(pp>i)
          ans+=(pp-i-1);
      }
      
      for(ll i=0;i<pos.size();i++)
      {
        ll pp= (upper_bound(pos.begin(),pos.end(),temp/pos[i])-pos.begin();
        if(pp>i)
        ans+=(pp-i-1);
      }
      //trace(mid,ans);
      if(ans==k)
        fin=mid;
      if(ans>=k)
      { 
        
        r= mid;
      }
      else
      {
        l=mid+1;
      }
    
    }
    cout<<fin<<endl;
    return 0;

  }
  
  
  
  

}

Compiler output:
a.cc: In function 'int main()':
a.cc:142:75: error: expected ')' before ';' token
  142 |         ll pp= (upper_bound(pos.begin(),pos.end(),temp/pos[i])-pos.begin();
      |                ~                                                          ^
      |                                                                           )