Language: C++

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<ll,ll> pl;
typedef pair<pl,int> pli;
typedef pair<pl,pl> pll;

ll N, M, Q, X, Y, Z, A, B, C, D, E, F, G;

int found = 0;

int check[100005];

int out_degree[100005];

vector<int> AdjList[200005];

vector<pli> EdgeList;

void dfs(int x){
	int last = 0, cnt = 0;
	for (auto it : AdjList[x]){
		if (EdgeList[it].second != 0) continue;
		cnt++;
		last = it;
	}
	if ((cnt+out_degree[x])%2==0){
		for (auto it : AdjList[x]){
			if (EdgeList[it].second != 0) continue;
			if (x == EdgeList[it].first.first) EdgeList[it].second = 1;
			else EdgeList[it].second = 2;
			out_degree[x]++;
		}
	}
	else{
		for (auto it : AdjList[x]){
			if (EdgeList[it].second != 0) continue;
			if (it == last){
				if (x == EdgeList[it].first.first) EdgeList[it].second = 2;
				else EdgeList[it].second = 1;
				out_degree[last]++;
			}
			if (x == EdgeList[it].first.first) EdgeList[it].second = 1;
			else EdgeList[it].second = 2;
			out_degree[x]++;
		}
		dfs(last);
	}
}

void dfs2(int x, int p){
	if (p != -1 && out_degree[x]%2==0){
		found = 1;
		return;
	}
	for (auto it : AdjList[x]){
		auto k = EdgeList[it];
		if (x == k.first){
			dfs2(k.second,x);
		}
		else{
			dfs2(k.first,x);
		}
		if (found){
			EdgeList[it].second = 3 - EdgeList[it].second;
			return;
		}
	}

}

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cin >> N >> M;
	if (M%2!=0){
		cout << "-1\n";
		return 0;
	}
	for (int i = 1; i <= M; ++i){
		cin >> A >> B;
		if (A > B) swap(A,B);
		EdgeList.push_back(pli(pl(A,B),0));
		AdjList[A].push_back(i-1);
		AdjList[B].push_back(i-1);
	}
	for (int i = 1; i <= N; ++i){
		if (!check[i]){
			dfs(i);
		}
	}
	for (int i = 1; i <= N; ++i){
		if (out_degree[i]%2!=0){
			found = 0;
			dfs2(i,-1);
		}
	}
	for (int i = 0; i < M; ++i){
		if (EdgeList[i].second == 1){
			cout << EdgeList[i].first.first << ' ' << EdgeList[i].first.second << '\n';
		}
		else cout << EdgeList[i].first.second << ' ' << EdgeList[i].first.first << '\n';
	}
}

Compiler output:
a.cc: In function 'void dfs2(int, int)':
a.cc:60:23: error: no match for 'operator==' (operand types are 'int' and 'std::pair<long long int, long long int>')
   60 |                 if (x == k.first){
      |                     ~ ^~ ~~~~~~~
      |                     |      |
      |                     int    std::pair<long long int, long long 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:1103:5: note: candidate: 'template<class _BiIter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const sub_match<_BiIter>&)'
 1103 |     operator==(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1103:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/c++/14/bits/regex.h:1199:5: 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>&)'
 1199 |     operator==(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1199:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/c++/14/bits/regex.h:1274:5: 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>&)'
 1274 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1274:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/c++/14/bits/regex.h:1366:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const typename std::iterator_traits<_Iter>::value_type*, const sub_match<_BiIter>&)'
 1366 |     operator==(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1366:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   'std::pair<long long int, long long int>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/c++/14/bits/regex.h:1441:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)'
 1441 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1441:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/c++/14/bits/regex.h:1534:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const typename std::iterator_traits<_Iter>::value_type&, const sub_match<_BiIter>&)'
 1534 |     operator==(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1534:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   'std::pair<long long int, long long int>' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/c++/14/bits/regex.h:1613:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator==(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)'
 1613 |     operator==(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1613:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/c++/14/bits/regex.h:2186:5: note: candidate: 'template<class _Bi_iter, class _Alloc> bool std::__cxx11::operator==(const match_results<_BiIter, _Alloc>&, const match_results<_BiIter, _Alloc>&)'
 2186 |     operator==(const match_results<_Bi_iter, _Alloc>& __m1,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:2186:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::__cxx11::match_results<_BiIter, _Alloc>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
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:1033:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator==(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1033 |     operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1033:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::pair<_T1, _T2>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/usr/include/c++/14/bits/stl_iterator.h:441:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  441 |     operator==(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:441:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/c++/14/bits/stl_iterator.h:486:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  486 |     operator==(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:486:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1667 |     operator==(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1667:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator==(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1737 |     operator==(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1737:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
In file included from /usr/include/c++/14/bits/char_traits.h:42,
                 from /usr/include/c++/14/string:42,
                 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/postypes.h:192:5: note: candidate: 'template<class _StateT> bool std::operator==(const fpos<_StateT>&, const fpos<_StateT>&)'
  192 |     operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/postypes.h:192:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::fpos<_StateT>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
In file included from /usr/include/c++/14/string:43:
/usr/include/c++/14/bits/allocator.h:235:5: note: candidate: 'template<class _T1, class _T2> bool std::operator==(const allocator<_CharT>&, const allocator<_T2>&)'
  235 |     operator==(const allocator<_T1>&, const allocator<_T2>&)
      |     ^~~~~~~~
/usr/include/c++/14/bits/allocator.h:235:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'const std::allocator<_CharT>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54:
/usr/include/c++/14/string_view:629:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator==(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  629 |     operator==(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:629:5: note:   template argument deduction/substitution failed:
a.cc:60:28: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
   60 |                 if (x == k.first){
      |                            ^~~~~
/usr/include/