code:

#include<bits/stdc++.h>
using namespace std;
map<int,set<int> >B;
int n,m,a,b;
priority_queue<pair<int,int>,vector<pair<int,int> > ,greater<pair<int,int> > >pq;
int main(){
    cin>>n>>m;
    for(int i=1;i<=n;i++){
        cin>>a>>b;
        B[a].insert(b);
        B[b].insert(a);
    }
    for(int i=1;i<=n;i++){
        if(B[i].size()==1){
            a=i;
            b=*B[i].begin();
            if(a>b)swap(a,b);
            pq.push({a,b});
        }
    }
    while(!pq.empty()){
        cout<<pq.top().first<<' '<<pq.top().second<<endl;
        pq.pop();
    }
}//ONLY AC #3.