#include<bits/stdc++.h>
using namespace std;
const int N = 5e5 + 5;
int n, sz, a[N];
vector < int > g[N];
void dfs(int v, int pr = -1){
for(int to : g[v]){
if(to != pr){
dfs(to, v);
}
}
if((int)g[v].size() == 1){
a[++sz] = v;
}
}
int main(){
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin >> n;
for(int i = 1; i < n; i++){
int x, y;
cin >> x >> y;
g[x].push_back(y);
g[y].push_back(x);
}
int rt = -1;
for(int i = 1; i <= n; i++){
if((int)g[i].size() > 1){
rt = i;
}
}
dfs(rt);
int ans = (sz + 1) / 2;
cout << ans << "\n";
for(int i = 1; i < ans; i++){
cout << a[i] << " " << a[i + ans] << "\n";
}
if(sz & 1){
cout << a[1] << " " << a[ans] << "\n";
}
else{
cout << a[sz] << " " << a[ans];
}
}
