Language: C++

#include<iostream>
#include<set>
#include<vector>
#include<algorithm>
using namespace std;

int main(){
	int n;
	cin >> n;
	
	set<int> dou;
	vector<int> can;
	
	int a;
	for(int i = 0; i < n; i++){
		cin >> a;
		if(dou.find(a) != dou.end()){
			can.push_back(a);
			dou.erase(dou.find(a));
		}else{
			dou.insert(a);
		}
	}
	
	if(can.size() < 2){
		cout << 0 << endl;
	}else{
		sort(dou.begin(), dou.end());
		cout << (long long)(dou[dou.size()-1]) * dou[dou.size()-2] << endl;
	}
	
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:29:40: error: no match for 'operator[]' (operand types are 'std::set<int>' and 'std::set<int>::size_type' {aka 'long unsigned int'})
   29 |                 cout << (long long)(dou[dou.size()-1]) * dou[dou.size()-2] << endl;
      |                                        ^
a.cc:29:61: error: no match for 'operator[]' (operand types are 'std::set<int>' and 'std::set<int>::size_type' {aka 'long unsigned int'})
   29 |                 cout << (long long)(dou[dou.size()-1]) * dou[dou.size()-2] << endl;
      |                                                             ^
In file included from /usr/include/c++/14/algorithm:61,
                 from a.cc:4:
/usr/include/c++/14/bits/stl_algo.h: In instantiation of 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = _Rb_tree_const_iterator<int>; _Compare = __gnu_cxx::__ops::_Iter_less_iter]':
/usr/include/c++/14/bits/stl_algo.h:4772:18:   required from 'void std::sort(_RAIter, _RAIter) [with _RAIter = _Rb_tree_const_iterator<int>]'
 4772 |       std::__sort(__first, __last, __gnu_cxx::__ops::__iter_less_iter());
      |       ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:28:7:   required from here
   28 |                 sort(dou.begin(), dou.end());
      |                 ~~~~^~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:1906:50: error: no match for 'operator-' (operand types are 'std::_Rb_tree_const_iterator<int>' and 'std::_Rb_tree_const_iterator<int>')
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
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:618:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr decltype ((__y.base() - __x.base())) std::operator-(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  618 |     operator-(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:618:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<int>' is not derived from 'const std::reverse_iterator<_Iterator>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1790:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr decltype ((__x.base() - __y.base())) std::operator-(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1790 |     operator-(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1790:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<int>' is not derived from 'const std::move_iterator<_IteratorL>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~