Language: C++

#include <bits/stdc++.h>
using namespace std;
typedef pair<ll, ll> P;

ll abs(ll n) {
  if (n < 0) {n = -n;}
  return n;
}

int main() {
  int N; 
  cin >> N;
  ll a[N];
  for (int i = 0; i < N; i++) {
    cin >> a[i];
  }
  vector<P> v;
  
  for (int i = 0; i < N; i++) {
    v.push_back(P(abs(a[i]), a[i]));
  }
  sort(v.begin(), v.end());
  
  int flag = 0;
  ll ans = 0;
  for (int i = N-1; i > 0; i--) {
    if (v[i].second < 0) {flag++;}
    ans += v[i].first;
  }
  
  if (flag % 2 == 0) {ans += v[0].second;}
  else {ans -= v[0].second;}
  
  cout << ans << endl;
}

Compiler output:
a.cc:3:14: error: 'll' was not declared in this scope
    3 | typedef pair<ll, ll> P;
      |              ^~
a.cc:3:18: error: 'll' was not declared in this scope
    3 | typedef pair<ll, ll> P;
      |                  ^~
a.cc:3:20: error: template argument 1 is invalid
    3 | typedef pair<ll, ll> P;
      |                    ^
a.cc:3:20: error: template argument 2 is invalid
a.cc:5:1: error: 'll' does not name a type
    5 | ll abs(ll n) {
      | ^~
a.cc: In function 'int main()':
a.cc:13:3: error: 'll' was not declared in this scope
   13 |   ll a[N];
      |   ^~
a.cc:15:12: error: 'a' was not declared in this scope
   15 |     cin >> a[i];
      |            ^
a.cc:20:23: error: 'a' was not declared in this scope
   20 |     v.push_back(P(abs(a[i]), a[i]));
      |                       ^
a.cc:20:34: error: expression list treated as compound expression in functional cast [-fpermissive]
   20 |     v.push_back(P(abs(a[i]), a[i]));
      |                                  ^
a.cc:25:5: error: expected ';' before 'ans'
   25 |   ll ans = 0;
      |     ^~~~
      |     ;
a.cc:27:14: error: request for member 'second' in 'v.std::vector<int>::operator[](((std::vector<int>::size_type)i))', which is of non-class type '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type' {aka 'int'}
   27 |     if (v[i].second < 0) {flag++;}
      |              ^~~~~~
a.cc:28:5: error: 'ans' was not declared in this scope; did you mean 'abs'?
   28 |     ans += v[i].first;
      |     ^~~
      |     abs
a.cc:28:17: error: request for member 'first' in 'v.std::vector<int>::operator[](((std::vector<int>::size_type)i))', which is of non-class type '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type' {aka 'int'}
   28 |     ans += v[i].first;
      |                 ^~~~~
a.cc:31:23: error: 'ans' was not declared in this scope; did you mean 'abs'?
   31 |   if (flag % 2 == 0) {ans += v[0].second;}
      |                       ^~~
      |                       abs
a.cc:31:35: error: request for member 'second' in 'v.std::vector<int>::operator[](0)', which is of non-class type '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type' {aka 'int'}
   31 |   if (flag % 2 == 0) {ans += v[0].second;}
      |                                   ^~~~~~
a.cc:32:9: error: 'ans' was not declared in this scope; did you mean 'abs'?
   32 |   else {ans -= v[0].second;}
      |         ^~~
      |         abs
a.cc:32:21: error: request for member 'second' in 'v.std::vector<int>::operator[](0)', which is of non-class type '__gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type' {aka 'int'}
   32 |   else {ans -= v[0].second;}
      |                     ^~~~~~
a.cc:34:11: error: 'ans' was not declared in this scope; did you mean 'abs'?
   34 |   cout << ans << endl;
      |           ^~~
      |           abs