Language: C++

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
constexpr long long MOD = 1000000007;
constexpr long long INF = 1LL << 60;
const long double PI = acosl(-1.0);
constexpr long double EPS = 1e-11;
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
int main(){
ll n;
cin>>n;
map<ll,ll>mp;
for(ll i=0;i<n;i++){
    ll t;
    cin >> t;
    mp[t]++;
}
ll ans = 0;
for (auto itr = mp.rbegin(); itr >= mp.rend();itr++){
    if(itr->second>=4){
        ans = (itr->second) * (itr->second);
        cout << ans << endl;
        return 0;
    } else if (itr->second>=2){
        for (auto itr2 = itr + 1; itr2 >= mp.rend();itr2++){
            if(itr2->second>=2){
                ans = (itr->second) * (itr2->second);
                cout <<ans<<endl;
                return 0;
            }
        }
    }
}
}

Compiler output:
In file included from /usr/include/c++/14/bits/stl_algobase.h:67,
                 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_iterator.h: In instantiation of 'constexpr std::reverse_iterator<_Iterator> std::reverse_iterator<_Iterator>::operator+(difference_type) const [with _Iterator = std::_Rb_tree_iterator<std::pair<const long long int, long long int> >; difference_type = long int]':
a.cc:38:32:   required from here
   38 |         for (auto itr2 = itr + 1; itr2 >= mp.rend();itr2++){
      |                                ^
/usr/include/c++/14/bits/stl_iterator.h:341:41: error: no match for 'operator-' (operand types are 'const std::_Rb_tree_iterator<std::pair<const long long int, long long int> >' and 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const long long int, long long int> > >::difference_type' {aka 'long int'})
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
/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_iterator.h:341:41: note:   'const std::_Rb_tree_iterator<std::pair<const long long int, long long int> >' is not derived from 'const std::reverse_iterator<_Iterator>'
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
/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_iterator.h:341:41: note:   'const std::_Rb_tree_iterator<std::pair<const long long int, long long int> >' is not derived from 'const std::move_iterator<_IteratorL>'
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
In file included from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127:
/usr/include/c++/14/complex:370:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&, const complex<_Tp>&)'
  370 |     operator-(const complex<_Tp>& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:370:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_iterator.h:341:41: note:   'const std::_Rb_tree_iterator<std::pair<const long long int, long long int> >' is not derived from 'const std::complex<_Tp>'
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
/usr/include/c++/14/complex:379:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&, const _Tp&)'
  379 |     operator-(const complex<_Tp>& __x, const _Tp& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:379:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_iterator.h:341:41: note:   'const std::_Rb_tree_iterator<std::pair<const long long int, long long int> >' is not derived from 'const std::complex<_Tp>'
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
/usr/include/c++/14/complex:388:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const _Tp&, const complex<_Tp>&)'
  388 |     operator-(const _Tp& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:388:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_iterator.h:341:41: note:   mismatched types 'const std::complex<_Tp>' and 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const long long int, long long int> > >::difference_type' {aka 'long int'}
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
/usr/include/c++/14/complex:465:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&)'
  465 |     operator-(const complex<_Tp>& __x)
      |     ^~~~~~~~
/usr/include/c++/14/complex:465:5: note:   candidate expects 1 argument, 2 provided
In file included from /usr/include/c++/14/valarray:605,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:166:
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom1, class _Dom2> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const _Expr<_Dom2, typename _Dom2::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_iterator.h:341:41: note:   'const std::_Rb_tree_iterator<std::pair<const long long int, long long int> >' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_Constant, _Dom, typename _Dom::value_type>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const typename _Dom::value_type&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_iterator.h:341:41: note:   'const std::_Rb_tree_iterator<std::pair<const long long int, long long int> >' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Constant, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const typename _Dom::value_type&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_iterator.h:341:41: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const long long int, long long int> > >::difference_type' {aka 'long int'}
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_ValArray, _Dom, typename _Dom::value_type>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const valarray<typename _Dom::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_iterator.h:341:41: note:   'const std::_Rb_tree_iterator<std::pair<const long long int, long long int> >' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const valarray<typename _Dom::value_type>&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_iterator.h:341:41: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'std::reverse_iterator<std::_Rb_tree_iterator<std::pair<const long long int, long long int> > >::difference_type' {aka 'long int'}
  341 |       { return reverse_iterator(current - __n); }
      |                                 ~~~~~~~~^~~~~
/usr/include/c++/14/valarray:1197:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__minus, _Tp>::result_type> std::operator-(const valarray<_Tp>&, const valarray<_Tp>&)'
 1197 | _DEFINE_BINARY_OPERATOR(-, __minus)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1197:1: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_iterator.h:341:41: note:   'const std::_Rb_tree_iterator<std::pair<const long long i