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2177.cpp
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2177.cpp
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#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5;
int n, m, vis, Time, num[N], low[N];
bool rem[N], hasbrid;
vector<pair<int, int>> g[N], ans;
void dfs(int u) {
num[u] = low[u] = ++Time;
++vis;
for (auto [v, id]: g[u]) {
if (rem[id]) continue;
rem[id] = true;
ans.emplace_back(u, v);
if (num[v]) {
low[u] = min(low[u], num[v]);
} else {
dfs(v);
low[u] = min(low[u], low[v]);
if (low[v] >= num[v]) {
hasbrid = true;
}
}
}
}
int main() {
ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin >> n >> m;
for (int i = 1, u, v; i <= m; ++i) {
cin >> u >> v;
g[u].emplace_back(v, i);
g[v].emplace_back(u, i);
}
dfs(1);
if (hasbrid || vis < n) {
cout << "IMPOSSIBLE\n";
} else {
for (auto [u, v]: ans) {
cout << u << " " << v << "\n";
}
}
return 0;
}