#include<cstdio>
#include<vector>
#include<algorithm>
using namespace std;
int n,m,order=0;
int low[20004],dfn[20004],father[20004],son[20004];
//father:父结点 son:子结点个数
vector<int> cutpoint,edge[20004];
vector< pair<int,int> > cutedge;
void tarjan(int u)
{
dfn[u]=low[u]=++order;
bool flag=false;
for (int i=0;i<edge[u].size();i++)
{
int v=edge[u][i];
if(!dfn[v])
{
son[u]++;
father[v]=u;
tarjan(v);
if(low[v]>=dfn[u]) flag=true;
//点u为割点
if(low[v]>dfn[u]) cutedge.push_back(make_pair(min(v,u),max(v,u)));
//边v-u为割边
low[u]=min(low[u],low[v]);
}
else if(v!=father[u]) low[u]=min(low[u],dfn[v]);
}
//根节点若有两棵或两棵以上的子树则该为割点
//非根节点若所有子树节点均没有指向u的祖先节点的回边则为割点
if((father[u]==0&&son[u]>1)||(father[u]&&flag)) cutpoint.push_back(u);
}
int main()
{
scanf("%d%d",&n,&m);
for (int i=1;i<=m;i++)
{
int u,v;
scanf("%d%d",&u,&v);
edge[u].push_back(v),edge[v].push_back(u);
}
tarjan(1);
sort(cutedge.begin(),cutedge.end());
sort(cutpoint.begin(),cutpoint.end());
if(0==cutpoint.size()) puts("Null");
else
{
printf("%d",cutpoint[0]);
for (int i=1;i<cutpoint.size();i++) printf(" %d",cutpoint[i]);
puts("");
}
for(int i=0;i<cutedge.size();i++) printf("%d %d\n",cutedge[i].first,cutedge[i].second);
}
