#include <bits/stdc++.h>
#define int int64_t
using namespace std;
vector<vector<int>>graph;
vector<int>dist;
vector<int>leaf;
void dfs(int v, int p){
dist[v] = dist[p] + 1;
if(graph[v].size() == 1) leaf.push_back(v);
for(auto u: graph[v]){
if(u != p) dfs(u, v);
}
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n; cin>>n;
graph.assign(n, vector<int>());
dist.assign(n, -1);
for(int i = 0; i<n-1; ++i){
int a, b; cin>>a>>b;
--a; --b;
graph[a].push_back(b);
graph[b].push_back(a);
}
dfs(0, 0);
int m = leaf.size();
/*
for(auto u: leaf) cout<<u<<" ";
cout<<endl;
for(auto u: dist) cout<<u<<" ";
cout<<endl;
*/
vector<pair<int, int>>fred;
for(auto u: leaf){
fred.push_back({dist[u], u});
}
sort(fred.begin(), fred.end());
/*
for(auto [x, y]: fred) cout<<y<<" ";
cout<<endl;
*/
cout<<(m+1)/2<<'\n';
for(int i = 0; i<m/2; ++i){
cout<<leaf[i]+1<<" "<<leaf[m/2+i]+1<<'\n';
}
if(leaf.size()%2) cout<<leaf[m/2]+1<<" "<<leaf[m-1]+1<<'\n';
//cout<<'\n';
}
