#include <bits/stdc++.h>
using namespace std;
int main() {
cin.tie(nullptr)->sync_with_stdio(false);
#ifdef palilo
freopen("in", "r", stdin);
freopen("out", "w", stdout);
#endif
int n;
cin >> n;
vector<vector<int>> adj(n);
for (int u, v, i = n - 1; i--;) {
cin >> u >> v, --u, --v;
adj[u].emplace_back(v);
adj[v].emplace_back(u);
}
const auto rev_dfn = [&](int root) {
vector<int> stk = {root}, dfn(n);
for (auto& u : dfn) {
u = stk.back();
stk.pop_back();
for (const auto& v : adj[u]) {
adj[v].erase(find(adj[v].begin(), adj[v].end(), u));
stk.emplace_back(v);
}
}
reverse(dfn.begin(), dfn.end());
return dfn;
}(0);
vector<pair<int, int>> ans;
vector dp(n, -1);
for (const auto& u : rev_dfn) {
for (const auto& v : adj[u]) {
if (~dp[v]) {
if (dp[u] == -1) {
dp[u] = dp[v];
} else {
ans.emplace_back(exchange(dp[u], -1), dp[v]);
}
}
}
if (dp[u] == -1) {
dp[u] = u;
}
}
if (dp[0]) {
ans.emplace_back(dp[0], 0);
}
cout << ans.size() << '\n';
for (const auto& [u, v] : ans) {
cout << u + 1 << ' ' << v + 1 << '\n';
}
}
