#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define dbg(...) 42
#endif
#define int long long
const int N = 5e5 + 1;
vector<int> adj[N];
vector<int> v;
void dfs(int u, int p) {
if (adj[u].size() == 1) {
v.push_back(u);
}
for (auto v : adj[u]) {
if (v != p) dfs(v, u);
}
}
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
for (int i = 1; i < n; i++) {
int a, b;
cin >> a >> b;
adj[a].push_back(b);
adj[b].push_back(a);
}
dfs(1, 0);
int m = v.size();
cout << (m + 1) / 2 << '\n';
for (int i = 0; i < m / 2; i++) {
cout << v[i] << ' ' << v[i + m / 2] << '\n';
}
if (m & 1) {
cout << v.back() << ' ' << v[0] << '\n';
}
return 0;
}
