Language: C++

#include <bits/stdc++.h>

#define all(v) v.begin(), v.end()
#define mp make_pair
#define EPS (int)1e-9
#define PI acos(-1)
#define endl '\n'
#define pb push_back
#define fi first
#define se second

using namespace std;
using ld = long double;
using ll = long long;
using vi = vector<int>;
using pii = pair<int, int>;

const int mod = 1000000007;


int main()
{
	ios_base::sync_with_stdio(0); cin.tie(0);
	int n, x;
	cin >> n;

	set<int> seen;
	vi a;
	for(int i = 0; i < n; i++)
	{
		cin >> x;
		if(seen.count(x))
		{
			a.pb(x);
			seen.erase(x);
		}
		else
			seen.insert(x);
	}

	sort(all(a));
	int n1 = a.size();

	if(a.size() <= 1)
		cout << 0 << endl;
	else
		cout << a[n1-1]*a[n1-2] < endl;

	return 0;	
}

Compiler output:
a.cc: In function 'int main()':
a.cc:47:41: error: no match for 'operator<' (operand types are 'std::basic_ostream<char>' and 'char')
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                         ^
a.cc:47:41: note: candidate: 'operator<(int, int)' (built-in)
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                         ^
a.cc:47:41: note:   no known conversion for argument 1 from 'std::basic_ostream<char>' to 'int'
In file included from /usr/include/c++/14/regex:68,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:181,
                 from a.cc:1:
/usr/include/c++/14/bits/regex.h:1143:5: note: candidate: 'template<class _BiIter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const sub_match<_BiIter>&)'
 1143 |     operator<(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1143:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
/usr/include/c++/14/bits/regex.h:1224:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&, const sub_match<_BiIter>&)'
 1224 |     operator<(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1224:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
/usr/include/c++/14/bits/regex.h:1317:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(const sub_match<_BiIter>&, __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&)'
 1317 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1317:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
/usr/include/c++/14/bits/regex.h:1391:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type*, const sub_match<_BiIter>&)'
 1391 |     operator<(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1391:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'char'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
/usr/include/c++/14/bits/regex.h:1485:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)'
 1485 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1485:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
/usr/include/c++/14/bits/regex.h:1560:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type&, const sub_match<_BiIter>&)'
 1560 |     operator<(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1560:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'char'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
/usr/include/c++/14/bits/regex.h:1660:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)'
 1660 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1660:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/usr/include/c++/14/bits/stl_pair.h:1045:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator<(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1045 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1045:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'const std::pair<_T1, _T2>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/usr/include/c++/14/bits/stl_iterator.h:448:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  448 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:448:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'const std::reverse_iterator<_Iterator>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  493 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'const std::reverse_iterator<_Iterator>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1694 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'const std::move_iterator<_IteratorL>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1760 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'const std::move_iterator<_IteratorL>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |                 cout << a[n1-1]*a[n1-2] < endl;
      |                                           ^~~~
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/string_view:673:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  673 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:673:5: note:   template argument deduction/substitution failed:
a.cc:7:14: note:   'std::basic_ostream<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
    7 | #define endl '\n'
      |              ^~~~
a.cc:47:43: note: in expansion of macro 'endl'
   47 |