#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> pi;
#define PUB push_back
#define fi first
#define se second
int N, root = 1, cnt = 0, tot_leaf = 0, first_leaf = -1;
vector<int> adjL[500005];
deque<pi> leaf[500005]; //{indeks, dist}
pi data[500005]; // {id, byk_leaf}
void find_root(){
for(int i=1;i<=N;i++){
if(adjL[i].size() > adjL[root].size()){
root = i;
}
}
}
void find_leaf(int prn, int pos, int dist){
if(adjL[pos].size() == 1){
leaf[cnt].PUB({pos, dist});
tot_leaf++;
data[cnt].se++;
return;
}
for(auto nxt:adjL[pos]){
if(nxt == prn) continue;
find_leaf(pos, nxt, dist+1);
}
}
bool cmp(pi a, pi b){
return a.se > b.se;
}
bool cmp_leaf(pi a, pi b){
return a.se < b.se;
}
void print_ans(){
int pos = 1, kanan = cnt-1;
cout << (tot_leaf+1)/2 << endl;
for(int i=0;i<cnt;i++){
if(data[i].se == 0) continue;
pos = max(pos, i+1);
while(data[i].se > 0 && data[pos].se > 0){
int a = leaf[data[i].fi].back().fi,
b = leaf[data[pos].fi].front().fi;
cout << a << " " << b << endl;
leaf[data[pos].fi].pop_front(); leaf[data[i].fi].pop_back();
data[pos].se--; data[i].se--;
pos++;
if(pos > kanan){
while(data[kanan].se == 0){
kanan--;
}
pos = i+1;
}
}
while(data[i].se > 0){
if(data[i].se == 1){
cout << leaf[data[i].fi].back().fi << " " << first_leaf << endl;
data[i].se--;
}else{
cout << leaf[data[i].fi].front().fi; leaf[data[i].fi].pop_front();
cout << " " << leaf[data[i].fi].back().fi << endl; leaf[data[i].fi].pop_back();
data[i].se -= 2;
}
}
}
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> N;
for(int i=0;i<N-1;i++){
int x, y;
cin >> x >> y;
adjL[x].PUB(y);
adjL[y].PUB(x);
}
find_root();
for(auto nxt:adjL[root]){
data[cnt] = {cnt, 0};
find_leaf(root, nxt, 1);
cnt++;
}
sort(data, data+cnt, cmp);
for(int i=0;i<cnt;i++){
if(!leaf[i].empty()) sort(leaf[i].begin(), leaf[i].end(), cmp_leaf);
}
first_leaf = leaf[data[0].fi].back().fi;
// cerr << "root: " << root << endl;
// cerr << "first: " << first_leaf << endl;
// for(int i=0;i<cnt;i++){
// cerr << data[i].se << " -> ";
// for(auto elm:leaf[data[i].fi]) cerr << " [" << elm.fi << "," << elm.se << "]";
// cerr << endl;
// }
print_ans();
return 0;
}
