#include <bits/stdc++.h>
using namespace std;
const int MXN = 5e5 + 10;
vector<int> G[MXN];
vector<pair<int, int>> deg_one;
int dep[MXN];
void dfs(int u, int p) {
for (int v : G[u]) {
if (v == p) continue;
dep[v] = dep[u] + 1;
dfs(v, u);
}
if (G[u].size() == 1) {
deg_one.emplace_back(dep[u], u);
}
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int n; cin >> n;
for (int i = 1; i <= n - 1; i++) {
int u, v; cin >> u >> v;
G[u].emplace_back(v);
G[v].emplace_back(u);
}
dfs(1, -1);
int l = 0, r = (int)deg_one.size() - 1;
vector<pair<int, int>> ans;
for (; l < r; l++, r--) {
auto [dep_u, u] = deg_one[l];
auto [dep_v, v] = deg_one[r];
ans.emplace_back(u, v);
}
if (l == r) {
ans.emplace_back(1, deg_one[l].second);
}
cout << ans.size() << "\n";
for (auto [u, v] : ans) {
cout << u << " " << v << "\n";
}
}
