Language: C++

#include <bits/stdc++.h>

using namespace std;

const int OO = 1e9;
const double EPS = 1e-9;

#define ndl cout << '\n'
#define sz(v) int(v.size())
#define pb push_back
#define mp make_pair
#define fs first
#define sc second
#define present(a, x) (a.find(x) != a.end())
#ifdef LOCAL
#define db(...) ({cout << "> Line " << __LINE__  \
		<< ": "; _db(#__VA_ARGS__, __VA_ARGS__);})
#define RNG() rng()
#else
#define db(...) true
#define RNG() true
#endif

template<class T>
void _db(const char *dbStr, T e) {
	cout << dbStr << " = " << e << endl;
}
template<class T, class... L>
void _db(const char *dbStr, T e, L... r) {
  while(*dbStr != ',') cout << *dbStr++;
  cout << " = " << e << ','; _db(dbStr + 1, r...);
}
template<class S, class T>
ostream& operator<<(ostream& o, const map<S, T>& v) {
	o << "["; int i = 0;
	for (const pair<S, T>& pr : v) o << (!i++ ? "" : ", ") << "{"
	<< pr.fs << " : " << pr.sc << "}"; return o << "]";
}
template<template <class, class...> class S, class T, class... L>
ostream& operator<<(ostream& o, const S<T, L...>& v) {
	o << "["; int i = 0;
	for (const auto& e : v) o << (!i++ ? "" : ", ") << e;
	return o << "]";
}
template<class S, class T>
ostream& operator<<(ostream& o, const pair<S, T>& pr) {
	return o << "(" << pr.fs << ", " << pr.sc << ")";
}
ostream& operator<<(ostream& o, const string& s) {
	for (const char& c : s) o << c;
	return o;
}

template<class T> using V = vector<T>;
template<class T> using VV = V<V<T>>;
template<class T> using VVV = VV<V<T>>;
using ll = long long;
using pii = pair<int, int>;
using vi = V<int>;
using vii = V<pii>;
using vvi = VV<int>;
using mii = map<int, int>;
using umii = unordered_map<int, int>;
using si = set<int>;
using usi = unordered_set<int>;

const long long MX_P = 1e18 + 5;
int NN, K, zeros;
V<ll> A, P, N;

bool good(ll mid) {
		ll cnt = 0;

		if (mid >= 0) {
			cnt += zeros * 1LL * (zeros - 1) / 2 + zeros * 1LL * (NN - zeros) + sz(P) * 1LL * sz(N);
			for (int i = 0; i < sz(N); ++i) {
				cnt += max(upper_bound(N.begin(), N.end(), mid / N[i]) - (N.begin() + i + 1), 0);
				int x = upper_bound(N.begin(), N.end(), mid / N[i]) - (N.begin() + i + 1);
				db(x, cnt, mid / N[i]);
			}
			for (int i = 0; i < sz(P); ++i) {
				cnt += max(upper_bound(P.begin(), P.end(), mid / P[i]) - (P.begin() + i + 1), 0);
			}
		} else {
			for (int i = 0; i < sz(P); ++i) {
//				for (int j = 0; j < sz(N); ++j) {
//					if (P[i] * N[j] >= abs(mid))
//						++cnt;
//				}
				cnt += N.end() - lower_bound(N.begin(), N.end(), (-mid + P[i] - 1) / P[i]);
			}
		}
		db(mid, cnt);
		return cnt >= K;
}

int main() {
	#ifdef LOCAL
	auto stTime = clock();
	freopen("in.txt", "r", stdin);
	mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
	#endif
	ios::sync_with_stdio(false);
	cout.precision(10);
	cin.tie(0);

	cin >> NN >> K;

	A.assign(NN, 0);


	for (int i = 0; i < NN; ++i)
		cin >> A[i];

	V<ll> Nr;

	for (int i = 0; i < NN; ++i) {
		zeros += A[i] == 0;
		if (A[i] < 0)
			N.pb(-A[i]);
		else if (A[i] > 0)
			P.pb(A[i]);
	}

	Nr = N;

	sort(N.begin(), N.end());
	sort(Nr.rbegin(), Nr.rend());
	sort(P.begin(), P.end());

	db(A, P, N);

	ll lo = -MX_P, hi = MX_P;

	while (hi - lo > 5) {
		ll mid = (lo + hi) / 2;
		if (good(mid))
			hi = mid;
		else
			lo = mid + 1;
	}

	for (ll i = lo; i <= hi; ++i) {
		if (good(i)) {
			cout << i << '\n';
			return 0;
		}
	}



	#ifdef LOCAL
	cout << "\n\n\nExecution time: " <<
		(clock() - stTime) * 1e3 / CLOCKS_PER_SEC << " ms" << endl;
	#endif
    return 0;
}


Compiler output:
a.cc: In function 'bool good(ll)':
a.cc:77:43: error: no matching function for call to 'max(__gnu_cxx::__normal_iterator<long long int*, std::vector<long long int, std::allocator<long long int> > >::difference_type, int)'
   77 |                                 cnt += max(upper_bound(N.begin(), N.end(), mid / N[i]) - (N.begin() + i + 1), 0);
      |                                        ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   template argument deduction/substitution failed:
a.cc:77:43: note:   deduced conflicting types for parameter 'const _Tp' ('long int' and 'int')
   77 |                                 cnt += max(upper_bound(N.begin(), N.end(), mid / N[i]) - (N.begin() + i + 1), 0);
      |                                        ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: 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:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   template argument deduction/substitution failed:
a.cc:77:43: note:   mismatched types 'std::initializer_list<_Tp>' and 'long int'
   77 |                                 cnt += max(upper_bound(N.begin(), N.end(), mid / N[i]) - (N.begin() + i + 1), 0);
      |                                        ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:82:43: error: no matching function for call to 'max(__gnu_cxx::__normal_iterator<long long int*, std::vector<long long int, std::allocator<long long int> > >::difference_type, int)'
   82 |                                 cnt += max(upper_bound(P.begin(), P.end(), mid / P[i]) - (P.begin() + i + 1), 0);
      |                                        ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   template argument deduction/substitution failed:
a.cc:82:43: note:   deduced conflicting types for parameter 'const _Tp' ('long int' and 'int')
   82 |                                 cnt += max(upper_bound(P.begin(), P.end(), mid / P[i]) - (P.begin() + i + 1), 0);
      |                                        ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note:   candidate expects 3 arguments, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   template argument deduction/substitution failed:
a.cc:82:43: note:   mismatched types 'std::initializer_list<_Tp>' and 'long int'
   82 |                                 cnt += max(upper_bound(P.begin(), P.end(), mid / P[i]) - (P.begin() + i + 1), 0);
      |                                        ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~