1 #include <map>
 2 #include <set>
 3 #include <cmath>
 4 #include <queue>
 5 #include <stack>
 6 #include <cstdio>
 7 #include <string>
 8 #include <vector>
 9 #include <cstdlib>
10 #include <cstring>
11 #include <sstream>
12 #include <iostream>
13 #include <algorithm>
14 #include <functional>
15 using namespace std;
16 #define rep(i,a,n) for (int i=a;i<n;i++)
17 #define per(i,a,n) for (int i=n-1;i>=a;i--)
18 #define all(x) (x).begin(),(x).end()
19 #define pb push_back
20 #define mp make_pair
21 #define lson l,m,rt<<1
22 #define rson m+1,r,rt<<1|1
23 typedef long long ll;
24 typedef vector<int> VI;
25 typedef pair<int, int> PII;
26 const ll MOD = 1e9 + 7;
27 const int INF = 0x3f3f3f3f;
28 const int MAXN = 2e4 + 7;
29 // head
30
31 int n, m, order = 0;
32 int low[MAXN], dfn[MAXN];
33 int par[MAXN], son[MAXN];
34 VI G[MAXN];
35 VI cutpoint;
36 vector<PII> cutedge;
37
38 void tarjan(int u) {
39     dfn[u] = low[u] = ++order;
40     bool flag = false;
41     rep(i, 0, G[u].size()) {
42         int v = G[u][i];
43         if (!dfn[v]) {
44             son[u]++;
45             par[v] = u;
46             tarjan(v);
47             if (low[v] >= dfn[u]) flag = true;
48             if (low[v] > dfn[u]) cutedge.pb(mp(min(v, u), max(v, u)));
49             low[u] = min(low[u], low[v]);
50         }
51         else if (v != par[u]) low[u] = min(low[u], dfn[v]);
52     }
53     if ((par[u] == 0 && son[u] > 1) || (par[u] && flag)) cutpoint.pb(u);
54 }
55
56 int main()
57 {
58     cin >> n >> m;
59     while (m--) {
60         int u, v;
61         scanf("%d%d", &u, &v);
62         G[u].pb(v);
63         G[v].pb(u);
64     }
65     tarjan(1);
66     sort(cutedge.begin(), cutedge.end());
67     sort(cutpoint.begin(), cutpoint.end());
68     if (cutpoint.size() == 0) cout << "Null" << endl;
69     else {
70         cout << cutpoint[0];
71         rep(i, 1, cutpoint.size()) printf(" %d", cutpoint[i]);
72         cout << endl;
73     }
74     rep(i, 0, cutedge.size())
75         printf("%d %d\n", cutedge[i].first, cutedge[i].second);
76     return 0;
77 }