#include <iostream>
#include <map>
using namespace std;
 
int main() {
	map < int ,pair<int,int>> back;
	map <int, bool> used;
	int a,b;
	while (cin >>a >> b){
		if (!back.count(a))
			back[a] = make_pair(b,0);
		else back[a] = make_pair( back[a].first,  b);
 
		if (!back.count(b))
			back[b] = make_pair(a,0);
		else back[b] = make_pair( back[b].first,  a);		
	}
	for (auto x : back){
		auto R = x.first;	
 
		while (!used[R]){
			used[R] = true;
			cout << R<<" ";
			if (!used[back[R].first])
				R = back[R].first;
			else R = back[R].second;
		}
		cout << endl;
	}
 
	return 0;
}

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MCAxCjMgMgo0IDAKMyA0CjIgMQo5IDgKMTAgOQo4IDEw

0 1
3 2
4 0
3 4
2 1
9 8
10 9
8 10