#include <bits/stdc++.h>
using namespace std;
const int MXN = 5e5 + 10;
vector<int> G[MXN];
vector<int> deg_one;
int dep[MXN];
void dfs(int u, int p) {
if (G[u].size() == 1) {
deg_one.emplace_back(u);
}
for (int v : G[u]) {
if (v == p) continue;
dep[v] = dep[u] + 1;
dfs(v, 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);
deg_one.emplace_back(1);
int ans = (deg_one.size() / 2);
cout << ans << "\n";
for (int i = 0; i < ans; i++) {
cout << deg_one[i] << " " << deg_one[i + ans] << "\n";
}
}
