#include <bits/stdc++.h>
using namespace std;
constexpr int N = 5e5;
vector<int> g[N];
int vis[N];
bitset<N> path;
set<int> connected;
int dfs(int u) {
path.set(u);
while (!g[u].empty()) {
int v = g[u].back();
g[v].erase(find(g[v].begin(), g[v].end(), u));
if (g[v].empty()) connected.erase(v);
g[u].pop_back();
if (g[u].empty()) connected.erase(u);
if (path.test(v)) {
path.reset(u);
cout << u + 1 << ' ';
return v;
}
int r = dfs(v);
if (r != u) {
path.reset(u);
cout << u + 1 << ' ';
return r;
}
else cout << u + 1 << '\n';
}
return -1;
}
signed main() {
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int n, m;
cin >> n >> m;
while (m--) {
int a, b;
cin >> a >> b;
g[--a].push_back(--b);
g[b].push_back(a);
}
for (int i = 0; i < n; i++) if (!g[i].empty()) connected.insert(i);
while (!connected.empty()) dfs(*connected.begin());
}
