#include <iostream>
#include <type_traits>
#include <vector>
#include <cstdint>
#include <algorithm>
#define PROBLEM "https://onlinejudge.u-aizu.ac.jp/courses/library/5/GRL/all/GRL_3_A"

using i32   = int32_t;
using i64   = int64_t;
using u32   = uint32_t;
using u64   = uint64_t;
using uint  = unsigned int;
using usize = std::size_t;
using ll    = long long;
using ull   = unsigned long long;
using ld    = long double;
template<typename T, usize n>
using arr = T (&)[n];
template<typename T, usize n>
using c_arr = const T (&)[n];
template<typename T>
T in()
{
    T v;
    return std::cin >> v, v;
}
template<typename T, typename Uint, usize n, usize i>
T in_v(typename std::enable_if<(i == n), c_arr<Uint, n>>::type) { return in<T>(); }
template<typename T, typename Uint, usize n, usize i>
auto in_v(typename std::enable_if<(i < n), c_arr<Uint, n>>::type& szs)
{
    const usize s = (usize)szs[i];
    std::vector<decltype(in_v<T, Uint, n, i + 1>(szs))> ans(s);
    for (usize j = 0; j < s; j++) { ans[j] = in_v<T, Uint, n, i + 1>(szs); }
    return ans;
}
template<typename T, typename Uint, usize n>
auto in_v(c_arr<Uint, n> szs) { return in_v<T, Uint, n, 0>(szs); }
template<typename... Types>
auto in_t() { return std::tuple<std::decay_t<Types>...>{in<Types>()...}; }

template<typename T>
int out(const T& v) { return std::cout << v, 0; }
template<typename T>
int out(const std::vector<T>& v)
{
    for (usize i = 0; i < v.size(); i++) {
        if (i > 0) { std::cout << ' '; }
        out(v[i]);
    }
    return std::cout << "\n", 0;
}
template<typename T1, typename T2>
int out(const std::pair<T1, T2>& v) { return out(v.first), std::cout << ' ', out(v.second), 0; }
template<typename T, typename... Args>
int out(const T& v, const Args... args) { return out(v), std::cout << ' ', out(args...), 0; }
template<typename... Args>
int outln(const Args... args) { return out(args...), std::cout << '\n', 0; }
template<typename... Args>
void outel(const Args... args) { return out(args...), std::cout << std::endl, 0; }


template<typename Cost = usize>
struct edge
{
    using cost_type = Cost;
    usize u, v;
    Cost c;
    edge(const usize u, const usize v) : u{u}, v{v}, c{1} {}
    edge(const usize u, const usize v, const Cost& c) : u{u}, v{v}, c{c} {}
    operator usize() { return v; }
    usize from() const { return u; }
    usize to() const { return v; }
    Cost cost() const { return c; }
    friend std::ostream& operator<<(std::ostream& os, const edge& e) { return os << e.u << "->" << e.v << ":" << e.c; }
};
template<typename Edge>
class base_graph
{
public:
    base_graph(const usize n) : v{n}, es(n), res(n) {}
    void add_edge(const usize u, const usize v, const bool bi = false)
    {
        es[u].emplace_back(u, v);
        if (bi) { es[v].emplace_back(v, u); }
    }
    template<typename Cost>
    void add_edge(const usize u, const usize v, const Cost& c, const bool bi = false)
    {
        es[u].emplace_back(u, v, c);
        if (bi) { es[v].emplace_back(v, u, c); }
    }
    std::vector<Edge>& operator[](const usize u) { return es[u]; }
    const std::vector<Edge>& operator[](const usize u) const { return es[u]; }
    std::vector<Edge>& from(const usize u) { return es[u]; }
    const std::vector<Edge>& from(const usize u) const { return es[u]; }
    std::vector<Edge>& to(const usize v) { return res[v]; }
    const std::vector<Edge>& to(const usize v) const { return res[v]; }
    usize size() const { return v; }

private:
    usize v;
    std::vector<std::vector<Edge>> es, res;
};
template<typename Edge>
using base_tree = base_graph<Edge>;
using graph     = base_graph<edge<>>;
using tree      = base_graph<edge<>>;
template<typename Cost>
using cost_graph = base_graph<edge<Cost>>;
template<typename Cost>
using cost_tree = base_graph<edge<Cost>>;
template<typename Edge = edge<>>
class lowlink_manager
{
private:
    using P = std::pair<usize, usize>;
    const usize sz;
    std::vector<P> bs;
    std::vector<usize> ord, low, as;
    std::vector<bool> is_a;
    std::vector<std::vector<usize>> dfs_tree;

public:
    lowlink_manager(const base_tree<Edge>& g) : sz(g.size()), ord(sz, sz), low(sz, sz), is_a(sz, false), dfs_tree(sz)
    {
        usize num = 0;
        auto dfs  = [&](auto&& self, const usize s, const usize p) -> void {
            ord[s] = low[s] = num++;
            usize back      = 0;
            for (const auto& e : g[s]) {
                const usize to = e.to();
                if (ord[to] == sz) {
                    dfs_tree[s].push_back(to), self(self, to, s), low[s] = std::min(low[s], low[to]);
                } else {
                    if (p == to) { back++; }
                    if (p != to or back > 1) { low[s] = std::min(low[s], ord[to]); }
                }
            }
        };
        auto dfs2 = [&](auto&& self, const usize s) -> void {
            usize max = 0;
            for (const usize to : dfs_tree[s]) {
                max = std::max(max, low[to]);
                if (this->is_bridge(s, to)) { bs.push_back(std::minmax(s, to)); }
                self(self, to);
            }
            is_a[s] = ord[s] <= max and (s != 0 or dfs_tree[s].size() > 1);
            if (is_a[s]) { as.push_back(s); }
        };
        for (usize i = 0; i < sz; i++) {
            if (ord[i] != sz) { continue; }
            dfs(dfs, i, sz), dfs2(dfs2, i);
        }
    }
    bool is_art(const usize i) const { return is_a[i]; }
    bool is_bridge(const usize i, const usize j) const { return (ord[i] < ord[j]) ? ord[i] < low[j] : ord[j] < low[i]; }
    const std::vector<P>& bridges() const { return bs; }
    const std::vector<usize>& arts() const { return as; }
};
int main()
{
    const auto V = in<usize>();
    const auto E = in<usize>();
    graph g(V);
    for (usize i = 0; i < E; i++) {
        const usize s = in<usize>();
        const usize t = in<usize>();
        g.add_edge(s, t, true);
    }
    auto bs = lowlink_manager<edge<>>{g}.bridges();
    std::sort(bs.begin(), bs.end());
    for (const auto& e : bs) { outln(e); }
    return 0;
}



Measured memory_bytes: 7720