//!yrt tsuj uoy srettam gnihton no emoc
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define pb push_back
#define mp make_pair
#define pii pair <int, int>
#define fast_io ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define F first
#define S second
const int maxn = 5e5 + 5;
vector <int> adj[maxn];
int st[maxn];
int tim = 0;
void dfs(int v, int p = -1){
st[v] = tim;
tim ++;
for(int u : adj[v]){
if(u != p){
dfs(u, v);
}
}
}
int main(){
fast_io;
int n;
cin >> n;
for(int i = 0; i < n - 1; i ++){
int x, y;
cin >> x >> y;
x --; y --;
adj[x].pb(y);
adj[y].pb(x);
}
int l = 0;
int r = 0;
for(int i = 0; i < n; i ++){
if((int)adj[i].size() == 1){
l ++;
}else{
r = i;
}
}
dfs(r);
vector <pii> vec;
for(int i = 0; i < n; i ++){
if((int)adj[i].size() == 1){
vec.pb({st[i], i + 1});
}
}
sort(vec.begin(), vec.end());
cout << (l + 1) / 2 << '\n';
for(int i = 0; i < l / 2; i ++){
cout << vec[i].S << " " << vec[i + l / 2].S << '\n';
}
if(l % 2){
cout << vec[l - 1].S << " " << vec[0].S << '\n';
}
return 0;
}
