#include "bits/stdc++.h"
using namespace std;
void abc() {cout << endl;}
template <typename T, typename ...U> void abc(T a, U ...b) {
cout << a << ' ', abc(b...);
}
template <typename T> void printv(T l, T r) {
while (l != r) cout << *l << " \n"[++l == r];
}
template <typename A, typename B> istream& operator >> (istream& o, pair<A, B> &a) {
return o >> a.first >> a.second;
}
template <typename A, typename B> ostream& operator << (ostream& o, pair<A, B> a) {
return o << '(' << a.first << ", " << a.second << ')';
}
template <typename T> ostream& operator << (ostream& o, vector<T> a) {
bool is = false;
for (T i : a) {o << (is ? ' ' : '{'), is = true, o << i;}
return o << '}';
}
#ifdef local
#define test(args...) abc("[" + string(#args) + "]", args)
#else
#define test(args...) void(0)
#endif
using ll = long long;
vector<int> adj[500005];
bool seen[500005];
vector<int> order;
void dfs(int node) {
seen[node] = true;
bool leaf = true;
for (auto nxt : adj[node]) {
if (seen[nxt]) continue;
dfs(nxt);
leaf = false;
}
if (leaf) order.push_back(node);
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
// freopen("", "r", stdin);
// freopen("", "w", stdout);
int n; cin >> n;
for (int i = 0; i < n-1; i++) {
int a, b; cin >>a >> b;
adj[a].push_back(b);
adj[b].push_back(a);
}
for (int i = 1; i <= n; i++) {
if (adj[i].size() >= 2) {
dfs(i);
break;
}
}
int s = order.size();
cout << ceil(s / 2.0) << endl;
for (int i = 0; i < (s + 1) /2; i++) {
cout << order[i] << " " << order[min(i + s/2, s-1)] << "\n";
}
}
