#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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