Language: C++

#include<iostream>
#include<vector>
#include<set>

int main(){
    int N, K;
    std::cin >> N >> K;
    std::vector< int > A(N);
    for(int i=0; i<N;i++){
        std::cin >> A[i];
    }
    int combin_N = ( N * (N - 1) ) / 2;
    
    std::set< long long > calc(combin_N);
    int cnt = 0;
    for(int i=0; i<N-1; i++){
        for(int j=i+1; j<N; j++){
            calc.insert( (long long)A[i] * (long long)A[j] );
            cnt++;
        }
    }
    
    auto itr = calc.begin() + K - 1;
    std::cout << *itr << std::endl;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:14:40: error: no matching function for call to 'std::set<long long int>::set(int&)'
   14 |     std::set< long long > calc(combin_N);
      |                                        ^
In file included from /usr/include/c++/14/set:63,
                 from a.cc:3:
/usr/include/c++/14/bits/stl_set.h:273:9: note: candidate: 'template<class _InputIterator> std::set<_Key, _Compare, _Alloc>::set(_InputIterator, _InputIterator, const allocator_type&) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>]'
  273 |         set(_InputIterator __first, _InputIterator __last,
      |         ^~~
/usr/include/c++/14/bits/stl_set.h:273:9: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:210:9: note: candidate: 'template<class _InputIterator> std::set<_Key, _Compare, _Alloc>::set(_InputIterator, _InputIterator, const _Compare&, const allocator_type&) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>]'
  210 |         set(_InputIterator __first, _InputIterator __last,
      |         ^~~
/usr/include/c++/14/bits/stl_set.h:210:9: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:193:9: note: candidate: 'template<class _InputIterator> std::set<_Key, _Compare, _Alloc>::set(_InputIterator, _InputIterator) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>]'
  193 |         set(_InputIterator __first, _InputIterator __last)
      |         ^~~
/usr/include/c++/14/bits/stl_set.h:193:9: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:267:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::set(std::initializer_list<_Tp>, const allocator_type&) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>; allocator_type = std::allocator<long long int>]'
  267 |       set(initializer_list<value_type> __l, const allocator_type& __a)
      |       ^~~
/usr/include/c++/14/bits/stl_set.h:267:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:261:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::set(std::set<_Key, _Compare, _Alloc>&&, std::__type_identity_t<_Alloc>&) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>; std::__type_identity_t<_Alloc> = std::allocator<long long int>]'
  261 |       set(set&& __x, const __type_identity_t<allocator_type>& __a)
      |       ^~~
/usr/include/c++/14/bits/stl_set.h:261:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:257:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::set(const std::set<_Key, _Compare, _Alloc>&, std::__type_identity_t<_Alloc>&) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>; std::__type_identity_t<_Alloc> = std::allocator<long long int>]'
  257 |       set(const set& __x, const __type_identity_t<allocator_type>& __a)
      |       ^~~
/usr/include/c++/14/bits/stl_set.h:257:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:253:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::set(const allocator_type&) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>; allocator_type = std::allocator<long long int>]'
  253 |       set(const allocator_type& __a)
      |       ^~~
/usr/include/c++/14/bits/stl_set.h:253:33: note:   no known conversion for argument 1 from 'int' to 'const std::set<long long int>::allocator_type&' {aka 'const std::allocator<long long int>&'}
  253 |       set(const allocator_type& __a)
      |           ~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_set.h:245:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::set(std::initializer_list<_Tp>, const _Compare&, const allocator_type&) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>; allocator_type = std::allocator<long long int>]'
  245 |       set(initializer_list<value_type> __l,
      |       ^~~
/usr/include/c++/14/bits/stl_set.h:245:40: note:   no known conversion for argument 1 from 'int' to 'std::initializer_list<long long int>'
  245 |       set(initializer_list<value_type> __l,
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_set.h:233:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::set(std::set<_Key, _Compare, _Alloc>&&) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>]'
  233 |       set(set&&) = default;
      |       ^~~
/usr/include/c++/14/bits/stl_set.h:233:11: note:   no known conversion for argument 1 from 'int' to 'std::set<long long int>&&'
  233 |       set(set&&) = default;
      |           ^~~~~
/usr/include/c++/14/bits/stl_set.h:225:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::set(const std::set<_Key, _Compare, _Alloc>&) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>]'
  225 |       set(const set&) = default;
      |       ^~~
/usr/include/c++/14/bits/stl_set.h:225:11: note:   no known conversion for argument 1 from 'int' to 'const std::set<long long int>&'
  225 |       set(const set&) = default;
      |           ^~~~~~~~~~
/usr/include/c++/14/bits/stl_set.h:178:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::set(const _Compare&, const allocator_type&) [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>; allocator_type = std::allocator<long long int>]'
  178 |       set(const _Compare& __comp,
      |       ^~~
/usr/include/c++/14/bits/stl_set.h:178:27: note:   no known conversion for argument 1 from 'int' to 'const std::less<long long int>&'
  178 |       set(const _Compare& __comp,
      |           ~~~~~~~~~~~~~~~~^~~~~~
/usr/include/c++/14/bits/stl_set.h:169:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::set() [with _Key = long long int; _Compare = std::less<long long int>; _Alloc = std::allocator<long long int>]'
  169 |       set() = default;
      |       ^~~
/usr/include/c++/14/bits/stl_set.h:169:7: note:   candidate expects 0 arguments, 1 provided
a.cc:23:29: error: no match for 'operator+' (operand types are 'std::set<long long int>::iterator' {aka 'std::_Rb_tree<long long int, long long int, std::_Identity<long long int>, std::less<long long int>, std::allocator<long long int> >::const_iterator'} and 'int')
   23 |     auto itr = calc.begin() + K - 1;
      |                ~~~~~~~~~~~~ ^ ~
      |                          |    |
      |                          |    int
      |                          std::set<long long int>::iterator {aka std::_Rb_tree<long long int, long long int, std::_Identity<long long int>, std::less<long long int>, std::allocator<long long int> >::const_iterator}
In file included from /usr/include/c++/14/string:48,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_iterator.h:627:5: note: candidate: 'template<class _Iterator> constexpr std::reverse_iterator<_Iterator> std::operator+(typename reverse_iterator<_Iterator>::difference_type, const reverse_iterator<_Iterator>&)'
  627 |     operator+(typename reverse_iterator<_Iterator>::difference_type __n,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:627:5: note:   template argument deduction/substitution failed:
a.cc:23:31: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
   23 |     auto itr = calc.begin() + K - 1;
      |                               ^
/usr/include/c++/14/bits/stl_iterator.h:1798:5: note: candidate: 'template<class _Iterator> constexpr std::move_iterator<_IteratorL> std::operator+(typename move_iterator<_IteratorL>::difference_type, const move_iterator<_IteratorL>&)'
 1798 |     operator+(typename move_iterator<_Iterator>::difference_type __n,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1798:5: note:   template argument deduction/substitution failed:
a.cc:23:31: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
   23 |     auto itr = calc.begin() + K - 1;
      |                               ^
In file included from /usr/include/c++/14/string:54:
/usr/include/c++/14/bits/basic_string.h:3598:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3598 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3598:5: note:   template argument deduction/substitution failed:
a.cc:23:31: note:   'std::set<long long int>::iterator' {aka 'std::_Rb_tree<long long int, long long int, std::_Identity<long long int>, std::less<long long int>, std::allocator<long long int> >::const_iterator'} is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   23 |     auto itr = calc.begin() + K - 1;
      |                               ^
/usr/include/c++/14/bits/basic_string.h:3616:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3616 |     operator+(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3616:5: note:   template argument deduction/substitution failed:
a.cc:23:31: note:   mismatched types 'const _CharT*' and 'std::_Rb_tree_const_iterator<long long int>'
   23 |     auto itr = calc.begin() + K - 1;
      |                               ^
/usr/include/