Language: C++

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, a, b) for(int i = a; i < b; i++)
#define rrep(i, a, b) for(int i = b - 1; i >= a; i--)
#define ALL(a) a.begin(), a.end()
#define pii pair<int,int>
#pragma GCC optimize("Ofast")
#define pcnt __builtin_popcount
#define buli(x) __builtin_popcountll(x)
#define pb push_back
#define mp make_pair
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );
#define isSquare(x) (sqrt(x)*sqrt(x) == x)
template<class T>bool chmax(T &a, const T &b) {if(a<b){a = b; return 1;} return 0; };
template<class T>bool chmin(T &a, const T &b) {if(a>b){a = b; return 1;} return 0; };
inline void IN(void){return;}
template <typename First, typename... Rest> void IN(First& first, Rest&... rest){cin >> first;IN(rest...);return;}
inline void OUT(void){cout << "\n";return;}
template <typename First, typename... Rest> void OUT(First first, Rest... rest){cout << first << " ";OUT(rest...);return;}
const double EPS = 1e-9;
const int mod = 1e9 + 7;
const int INF = 1e9;
const long long LLINF = 1e18;
long long lcm(ll a, ll b){return a * b / __gcd(a,b);}
struct IoSetup { IoSetup() {
    cin.tie(nullptr);ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
} } iosetup;
template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 >& p) {
  os << p.first << " " << p.second;
  return os;
}
template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}
template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}
template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}
template <typename T> void Exit(T first){cout << first << endl;exit(0); };
template< int mod > struct ModInt {
    int x; ModInt() : x(0) {}
    ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}
    ModInt &operator+=(const ModInt &p) {if((x += p.x) >= mod) x -= mod;return *this;}
    ModInt &operator-=(const ModInt &p) {if((x += mod - p.x) >= mod) x -= mod;return *this;}
    ModInt &operator*=(const ModInt &p) {x = (int) (1LL * x * p.x % mod);return *this;}
    ModInt &operator/=(const ModInt &p) {*this *= p.inverse();return *this;}
    ModInt operator-() const { return ModInt(-x); }
    ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
    ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
    ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
    ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }
    bool operator==(const ModInt &p) const { return x == p.x; }
    bool operator!=(const ModInt &p) const { return x != p.x; }
    ModInt inverse() const {int a = x, b = mod, u = 1, v = 0, t;
    while(b > 0) { t = a / b; swap(a -= t * b, b); swap(u -= t * v, v); }return ModInt(u);}
    ModInt pow(int64_t n) const {ModInt ret(1), mul(x); while(n > 0) {if(n & 1) ret *= mul;mul *= mul;n >>= 1;}return ret;}
    friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x;}
    friend istream &operator>>(istream &is, ModInt &a) { int64_t t; is >> t; a = ModInt< mod >(t); return (is); }
    static int get_mod() { return mod; }
}; using modint = ModInt< mod >;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

template< typename T > struct edge {
   int src, to; T cost;
   edge(int to, T cost) : src(-1), to(to), cost(cost) {}
   edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
   edge &operator=(const int &x) { to = x; return *this;}
   operator int() const { return to; }
};
template< typename T > using Edges = vector< edge< T > >;
template< typename T > using WeightedGraph = vector< Edges< T > >;
using UnWeightedGraph = vector< vector< int > >;
template< typename T > using Matrix = vector< vector< T > >;

class UnionFind{
    public:
        vector<int> rank, p, num;

        UnionFind(){}
        UnionFind(int size){
            rank.resize(size, 0);
            p.resize(size, 0);
            num.resize(size, 0);
            for(int i = 0; i < size; i++) makeSet(i);
        }

        void makeSet(int x){
            p[x] = x;
            rank[x] = 0;
            num[x] = 1;
        }//グループを作る

        bool same(int x, int y){
            return findSet(x) == findSet(y); //x,yが同じグループに属しているか判定
        }

        void unite(int x, int y){
            link(findSet(x), findSet(y));
        }//xとyの属するsetを結合する

        void link(int x, int y){
            if(rank[x] > rank[y]){
                p[y] = x;
                num[x] += num[y];
                num[y] = 0;
            }else{
                p[x] = y;
                if( rank[x] == rank[y]){
                    rank[y]++;
                }
                num[y] += num[x];
                num[x] = 0;
            }
        }//rankが大きい方の下にrankが小さいものをくっつける

        int findSet(int x){
            if(x != p[x]){
                p[x] = findSet(p[x]);
            }
            return p[x];
        }//再帰的にxの親を見つけている

        int size(int x){
            return num[findSet(x)];
        }// xの属する集合の大きさを返す
};

int N, M; 
vector<vector<int>> tree, ans;
// vector<pii> E;

void dfs(int v, int p = -1){
  // cerr << v << endl;
  for(auto nv: tree[v]){
    if(nv == p) continue;
    dfs(nv, v);
  }
  if(int(ans[v].size())%2 == 1) ans[v].push_back(p);
  else if(p != -1) ans[p].push_back(v);
}

int main(){
    iosetup;
    cin >> N >> M;
    UnionFind uf(N);
    tree.resize(N), ans.resize(N);
    // int cnt = 0;
    rep(i, 0, M){
        int u, v; cin >> u >> v; u--, v--;
        if(!uf.same(u, v)){
          uf.unite(u, v);
          tree[u].push_back(v);
          tree[v].push_back(u);
          // cnt++;
        }else{
          ans[u].push_back(v);
        }
    }
    cerr << cnt << endl;
    if(M % 2 == 1) Exit(-1);
    dfs(0);
    rep(i, 0, N){
      for(auto e: ans[i]){
        cout << i + 1 << " " << e + 1 << endl;
      }
    }

    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:174:13: error: 'cnt' was not declared in this scope; did you mean 'int'?
  174 |     cerr << cnt << endl;
      |             ^~~
      |             int