#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>
using namespace std;
#define endl '\n'
#define pi pair<int, int>
const int maxn = 500000;
int n;
vector<int> graph[maxn];
vector<int> lf;
void dfs(int c, int p){
if(graph[c].size() == 1) lf.push_back(c + 1);
for(int i : graph[c]){
if(i == p) continue;
dfs(i, c);
}
}
int main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
cin >> n;
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, -1);
int k = (lf.size() + 1) / 2;
cout << k << endl;
for(int i = 0; i < k; i++) cout << lf[i] << " " << lf[i + lf.size() / 2] << endl;
return 0;
}
