#include <bits/stdc++.h>
using namespace std;
vector<vector<int>> adj(500003);
vector<int> margini;
void dfs(int node, int parent){
if(adj[node].size() == 1){
margini.push_back(node);
}
for(int i : adj[node]){
if(i != parent){
dfs(i, node);}
}
}
int main(){
int n; cin >> n;
int a, b;
for(int i = 0; i < n - 1; i++){
cin >> a >> b;
adj[a].push_back(b);
adj[b].push_back(a);
}
dfs(1, 0);
int marime = margini.size();
int middle = marime / 2;
cout << marime / 2 + marime % 2 << '\n';
for(int i = 0; i < middle; i++){
cout << margini[i] << ' ' << margini[i + middle] << '\n';
}
if(marime % 2 == 1){
cout << margini[0] << ' ' << margini[marime - 1];
}
}
