Language: C++

#include <bits/stdc++.h>
using namespace std;

int main() {
	int n,sum=0;
	cin>>n;
	vector<int>vec(n);
	for(int i=0;i<n;i++){
		int a;
		cin>>a;
		vec.at(i)=a;
	}
	sort(vec.begin(),vec.end());
	reverse(vec.begin(),vec.end());
	if(n%2==0){
		for(int i=0;i<n/2;i++){
			sum+=vec.at(2i)-vec.at(2i+1);
		}
		cout<<sum<<endl;
	}
	else{
			for(int i=0;i<(n-1)/2;i++){
			sum+=vec.at(2i)-vec.at(2i+1);
		}
		cout<<sum+vec.at(n-1)<<endl;
	}
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:17:36: error: no matching function for call to 'std::vector<int>::at(std::complex<double>)'
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                              ~~~~~~^~~~
In file included from /usr/include/c++/14/vector:66,
                 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/stl_vector.h:1180:7: note: candidate: 'std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::at(size_type) [with _Tp = int; _Alloc = std::allocator<int>; reference = int&; size_type = long unsigned int]'
 1180 |       at(size_type __n)
      |       ^~
/usr/include/c++/14/bits/stl_vector.h:1180:20: note:   no known conversion for argument 1 from 'std::complex<double>' to 'std::vector<int>::size_type' {aka 'long unsigned int'}
 1180 |       at(size_type __n)
      |          ~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_vector.h:1199:7: note: candidate: 'std::vector<_Tp, _Alloc>::const_reference std::vector<_Tp, _Alloc>::at(size_type) const [with _Tp = int; _Alloc = std::allocator<int>; const_reference = const int&; size_type = long unsigned int]'
 1199 |       at(size_type __n) const
      |       ^~
/usr/include/c++/14/bits/stl_vector.h:1199:20: note:   no known conversion for argument 1 from 'std::complex<double>' to 'std::vector<int>::size_type' {aka 'long unsigned int'}
 1199 |       at(size_type __n) const
      |          ~~~~~~~~~~^~~
a.cc:17:50: error: no match for 'operator+' (operand types are 'std::complex<double>' and 'int')
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                                                ~~^~
      |                                                |  |
      |                                                |  int
      |                                                std::complex<double>
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:
/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:17:51: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
   17 |                         sum+=vec.at(2i)-vec.at(2i+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:17:51: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                                                   ^
In file included 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/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:17:51: note:   'std::complex<double>' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   17 |                         sum+=vec.at(2i)-vec.at(2i+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:17:51: note:   mismatched types 'const _CharT*' and 'std::complex<double>'
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                                                   ^
/usr/include/c++/14/bits/basic_string.h:3635:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(_CharT, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3635 |     operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Alloc>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3635:5: note:   template argument deduction/substitution failed:
a.cc:17:51: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'int'
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                                                   ^
/usr/include/c++/14/bits/basic_string.h:3652:5: 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 _CharT*)'
 3652 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3652:5: note:   template argument deduction/substitution failed:
a.cc:17:51: note:   'std::complex<double>' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                                                   ^
/usr/include/c++/14/bits/basic_string.h:3670:5: 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>&, _CharT)'
 3670 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs, _CharT __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3670:5: note:   template argument deduction/substitution failed:
a.cc:17:51: note:   'std::complex<double>' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                                                   ^
/usr/include/c++/14/bits/basic_string.h:3682:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3682 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3682:5: note:   template argument deduction/substitution failed:
a.cc:17:51: note:   'std::complex<double>' is not derived from 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                                                   ^
/usr/include/c++/14/bits/basic_string.h:3689:5: 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>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3689 |     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3689:5: note:   template argument deduction/substitution failed:
a.cc:17:51: note:   'std::complex<double>' is not derived from 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                                                   ^
/usr/include/c++/14/bits/basic_string.h:3696:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(__cxx11::basic_string<_CharT, _Traits, _Allocator>&&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3696 |     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3696:5: note:   template argument deduction/substitution failed:
a.cc:17:51: note:   'std::complex<double>' is not derived from 'std::__cxx11::basic_string<_CharT, _Traits, _Allocator>'
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                                                   ^
/usr/include/c++/14/bits/basic_string.h:3719:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(const _CharT*, __cxx11::basic_string<_CharT, _Traits, _Allocator>&&)'
 3719 |     operator+(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3719:5: note:   template argument deduction/substitution failed:
a.cc:17:51: note:   mismatched types 'const _CharT*' and 'std::complex<double>'
   17 |                         sum+=vec.at(2i)-vec.at(2i+1);
      |                                                   ^
/usr/include/c++/14/bits/basic_string.h:3726:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::__cxx11::basic_string<_CharT, _Traits, _Allocator> std::operator+(_CharT, __cxx11::b