Language: C++

#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  long K;
  cin >> N >> K;
  
  vector<long> A(N);
  vector<vector<long>> T(N, vector<long>(N,0) );
  
  for(int i=0; i<N; i++)
    cin >>  A.at(i);
  
  for(int i=0; i<N-1; i++){
    for(int j=i+1; j<N; j++)
      T.at( ( N*i - i*(i+1)/2 ) -1-i+j ) = A.at(i) * A.at(j);
  }
  
  sort(T.begin(), T.end());
  
  cout << T.at(K-1) << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:16:60: error: no match for 'operator=' (operand types are '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'} and '__gnu_cxx::__alloc_traits<std::allocator<long int>, long int>::value_type' {aka 'long int'})
   16 |       T.at( ( N*i - i*(i+1)/2 ) -1-i+j ) = A.at(i) * A.at(j);
      |                                                            ^
In file included from /usr/include/c++/14/vector:72,
                 from /usr/include/c++/14/functional:64,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:53,
                 from a.cc:1:
/usr/include/c++/14/bits/vector.tcc:210:5: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(const std::vector<_Tp, _Alloc>&) [with _Tp = long int; _Alloc = std::allocator<long int>]'
  210 |     vector<_Tp, _Alloc>::
      |     ^~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/vector.tcc:211:42: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<long int>, long int>::value_type' {aka 'long int'} to 'const std::vector<long int>&'
  211 |     operator=(const vector<_Tp, _Alloc>& __x)
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from /usr/include/c++/14/vector:66:
/usr/include/c++/14/bits/stl_vector.h:766:7: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::vector<_Tp, _Alloc>&&) [with _Tp = long int; _Alloc = std::allocator<long int>]'
  766 |       operator=(vector&& __x) noexcept(_Alloc_traits::_S_nothrow_move())
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:766:26: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<long int>, long int>::value_type' {aka 'long int'} to 'std::vector<long int>&&'
  766 |       operator=(vector&& __x) noexcept(_Alloc_traits::_S_nothrow_move())
      |                 ~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:788:7: note: candidate: 'std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::initializer_list<_Tp>) [with _Tp = long int; _Alloc = std::allocator<long int>]'
  788 |       operator=(initializer_list<value_type> __l)
      |       ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:788:46: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<long int>, long int>::value_type' {aka 'long int'} to 'std::initializer_list<long int>'
  788 |       operator=(initializer_list<value_type> __l)
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
a.cc:21:8: error: no match for 'operator<<' (operand types are 'std::ostream' {aka 'std::basic_ostream<char>'} and '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'})
   21 |   cout << T.at(K-1) << endl;
      |   ~~~~ ^~ ~~~~~~~~~
      |   |           |
      |   |           __gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type {aka std::vector<long int>}
      |   std::ostream {aka std::basic_ostream<char>}
In file included from /usr/include/c++/14/istream:41,
                 from /usr/include/c++/14/sstream:40,
                 from /usr/include/c++/14/complex:45,
                 from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127:
/usr/include/c++/14/ostream:116:7: note: candidate: 'std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(__ostream_type& (*)(__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]'
  116 |       operator<<(__ostream_type& (*__pf)(__ostream_type&))
      |       ^~~~~~~~
/usr/include/c++/14/ostream:116:36: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'} to 'std::basic_ostream<char>::__ostream_type& (*)(std::basic_ostream<char>::__ostream_type&)' {aka 'std::basic_ostream<char>& (*)(std::basic_ostream<char>&)'}
  116 |       operator<<(__ostream_type& (*__pf)(__ostream_type&))
      |                  ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/ostream:125:7: note: candidate: 'std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(__ios_type& (*)(__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>; __ios_type = std::basic_ios<char>]'
  125 |       operator<<(__ios_type& (*__pf)(__ios_type&))
      |       ^~~~~~~~
/usr/include/c++/14/ostream:125:32: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'} to 'std::basic_ostream<char>::__ios_type& (*)(std::basic_ostream<char>::__ios_type&)' {aka 'std::basic_ios<char>& (*)(std::basic_ios<char>&)'}
  125 |       operator<<(__ios_type& (*__pf)(__ios_type&))
      |                  ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
/usr/include/c++/14/ostream:135:7: note: candidate: 'std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]'
  135 |       operator<<(ios_base& (*__pf) (ios_base&))
      |       ^~~~~~~~
/usr/include/c++/14/ostream:135:30: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'} to 'std::ios_base& (*)(std::ios_base&)'
  135 |       operator<<(ios_base& (*__pf) (ios_base&))
      |                  ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
/usr/include/c++/14/ostream:174:7: note: candidate: 'std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char; _Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]'
  174 |       operator<<(long __n)
      |       ^~~~~~~~
/usr/include/c++/14/ostream:174:23: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'} to 'long int'
  174 |       operator<<(long __n)
      |                  ~~~~~^~~
/usr/include/c++/14/ostream:178:7: note: candidate: 'std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]'
  178 |       operator<<(unsigned long __n)
      |       ^~~~~~~~
/usr/include/c++/14/ostream:178:32: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'} to 'long unsigned int'
  178 |       operator<<(unsigned long __n)
      |                  ~~~~~~~~~~~~~~^~~
/usr/include/c++/14/ostream:182:7: note: candidate: 'std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char; _Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]'
  182 |       operator<<(bool __n)
      |       ^~~~~~~~
/usr/include/c++/14/ostream:182:23: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'} to 'bool'
  182 |       operator<<(bool __n)
      |                  ~~~~~^~~
In file included from /usr/include/c++/14/ostream:1022:
/usr/include/c++/14/bits/ostream.tcc:96:5: note: candidate: 'std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char; _Traits = std::char_traits<char>]'
   96 |     basic_ostream<_CharT, _Traits>::
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/ostream.tcc:97:22: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'} to 'short int'
   97 |     operator<<(short __n)
      |                ~~~~~~^~~
/usr/include/c++/14/ostream:189:7: note: candidate: 'std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]'
  189 |       operator<<(unsigned short __n)
      |       ^~~~~~~~
/usr/include/c++/14/ostream:189:33: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'} to 'short unsigned int'
  189 |       operator<<(unsigned short __n)
      |                  ~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/ostream.tcc:110:5: note: candidate: 'std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char; _Traits = std::char_traits<char>]'
  110 |     basic_ostream<_CharT, _Traits>::
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/ostream.tcc:111:20: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<std::vector<long int> >, std::vector<long int> >::value_type' {aka 'std::vector<long int>'} to 'int'
  111 |     operator<<(int __n)
      |                ~~~~^~~
/usr/include/c++/14/ostream:200:7: note: candidate: 'std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]'
  200 |       operator<<(unsigned int __n)
      |       ^~~~~~~~
/usr/include/c++/14/ostream:200:31: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_t